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Alluvial fans of post-glacial environments within British Columbia Ryder, June Margaret 1969

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ALLUVIAL FANS OF POST-GLACIAL ENVIRONMENTS WITHIN BRITISH COLUMBIA by j JUNE MARGARET RYDER B.Sc. U n i v e r s i t y of S h e f f i e l d , 1960 M.A. McMaster U n i v e r s i t y , 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of GEOGRAPHY We accept t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA December, 1969 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8 , Canada ABSTRACT A l l u v i a l f a n c o n s t r u c t i o n w i t h i n B r i t i s h Columbia was dependant upon temporary c o n d i t i o n s r e s u l t i n g from d e g l a c i a t i o n ; t h e r e i s no s i g n i f i c a n t f a n a g g r a d a t i o n at p r e s e n t . F i v e study areas were s e l e c t e d from the s e m i - a r i d s e c t i o n s of the F r a s e r , Thompson, B o n a p a r t e , South Thompson and Similkameen v a l l e y s . The T e r t i a r y and Quaternary geo-morphic h i s t o r i e s of these areas are s i m i l a r i n many r e s p e c t s - - m o s t s i g n i f i c a n t l y , one or more phases of P l e i s t o c e n e g l a c i a t i o n were f o l l o w e d by f l u v i a l and l a c u s -t r i n e a g g r a d a t i o n - - b u t v a r y r e g a r d i n g the amount of subsequent d o w n c u t t i n g by major r i v e r s . T h i s ranges from s e v e r a l hundred f e e t i n the F r a s e r and Thompson v a l l e y s to a few f e e t or none i n the Similkameen and Bonaparte v a l l e y s . S t r a t i g r a p h i c e v i d e n c e from the F r a s e r V a l l e y i n d i -c a t e s t h a t f a n b u i l d i n g commenced soon a f t e r the v a l l e y f l o o r became i c e - f r e e , p r o b a b l y w h i l s t g l a c i a l c o n d i t i o n s p e r s i s t e d i n t r i b u t a r y b a s i n s . I t c o n t i n u e d d u r i n g aggrada-t i o n by major r i v e r s and f o r sometime a f t e r w a r d s . In the Thompson and South Thompson v a l l e y s f a n s were most r e c e n t l y b u i l t upon d e g r a d a t i o n a l r i v e r t e r r a c e s . The o c c u r r e n c e of Mazama v o l c a n i c ash w i t h i n f a n s i n d i c a t e s t h a t c o n s t r u c t i o n c o n t i n u e d u n t i l a f t e r 6,600 y e a r s B.P. Fans were b u i l t d u r i n g a phase of l a n d s c a p e r e a d j u s t -ment from p r e d o m i n a n t l y g l a c i a l t o p r e d o m i n a n t l y f l u v i a l c o n d i t i o n s . They r e s u l t e d from the secondary d e p o s i t i o n of g l a c i a l d r i f t and l o c a l l y weathered m a t e r i a l by streams and mudflows. Fan c o m p o s i t i o n was dependant upon the n a t u r e of the a v a i l a b l e m a t e r i a l and upon the c h a r a c t e r of the p a r e n t b a s i n . For example, the widespread o c c u r r e n c e of g l a c i o - l a c u s t r i n e s i l t i n the Thompson V a l l e y gave r i s e t o fans composed of s i l t y mudflow g r a v e l s . G e n e r a l l y , s m a l l , steep b a s i n s produced mudflows whereas l a r g e r b a s i n s had more c o n s t a n t l y f l o w i n g streams which d e p o s i t e d f l u v i a l g r a v e l s . Fan a g g r a d a t i o n d e c l i n e d as the d r i f t s u p p l y was e x h a u s t e d ; d e p o s i t i o n of m a t e r i a l d e r i v e d by c u r r e n t w e a t h e r i n g was i n s u f f i c i e n t to m a i n t a i n the growth of the fans . A f t e r d e p o s i t i o n ceased many fans were d i s s e c t e d as a r e s u l t of l o c a l b a s e - l e v e l l o w e r i n g c o n t r o l l e d by d e g r a d a t i o n of major r i v e r s and/or fan-head t r e n c h i n g i n i -t i a t e d as the d e b r i s s u p p l y d e c l i n e d . Fan-head t r e n c h i n g i s b e s t developed i n the South Thompson V a l l e y ; b a s e - l e v e l d i s s e c t i o n predominates i n the F r a s e r and Thompson V a l l e y s . Where f a n b u i l d i n g p e r s i s t e d d u r i n g d e g r a d a t i o n , m u l t i -l e v e l f a n s were c o n s t r u c t e d . S t a t i s t i c a l c o r r e l a t i o n s among morphometric p a r a -meters d e s c r i b i n g f a n s and r e l a t e d b a s i n s i n d i c a t e t h a t b a s i n c h a r a c t e r i s t i c s e x e r t e d an i n f l u e n c e upon f a n geometry through the n a t u r e of the f a n b u i l d i n g stream. There r e l a t i o n s h i p s v a r y r e g i o n a l l y , p o s s i b l y r e f l e c t i n g l i t h o l o g i c , c l i m a t i c and geomorphic c o n t r a s t s . B r i t i s h Columbia fans are s t e e p e r and d i s p l a y a g r e a t e r v a r i a t i o n of morphometric r e l a t i o n s h i p s than fans of the a r i d Ameri-can Southwest. Fans r e s u l t i n g from d e g l a c i a t i o n are d i s t i n g u i s h e d by use of the p r e f i x " p a r a - g l a c i a l " . V TABLE OF CONTENTS CHAPTER PAGE INTRODUCTION 1 PART ONE. THE PHYSIOGRAPHIC HISTORY OF SELECTED REGIONS IN SOUTH CENTRAL BRITISH COLUMBIA ^ 1 THE PHYSIOGRAPHIC HISTORY OF SOUTH-CENTRAL BRITISH COLUMBIA - A GENERAL OUTLINE 5 1.1 Development of Landforms B e f o r e the P l e i s t o c e n e G l a c i a t i o n 5 1.1.1 The P r e - G l a c i a l F r a s e r V a l l e y . . 8 1.2 The P l e i s t o c e n e G l a c i a t i o n 1 1 1.2.1 Sequence of G l a c i a t i o n s on the Coast 1 2 1.2.2 Sequence of G l a c i a t i o n s IncLand . . 1 5 1.2.3 C o r r e l a t i o n of G l a c i e r F l u c t u a t i o n s ^ 9 1.2.4 Ice D i s p e r s a l and I c e Movement . . 21 1.2.5 Ice T h i c k n e s s 2 2 1.2.6 P o s t - G l a c i a l H i s t o r y 2 3 2 THE PLEISTOCENE GLACIATION OF STUDY AREAS . . 24 2.1 G l a c i a l E f f e c t s w i t h i n the Study Area s . . 2 4 2.1.1 The F r a s e r V a l l e y 24 2.1.2 The Thompson V a l l e y 2 5 2.1.3 The Bonaparte V a l l e y 2 7 2.1.4 The Similkameen V a l l e y . 2 7 2.1.5 The Kamloops Area 2 9 2.2 D e g l a c i a t i o n of the Fraser-Thompson Drainage System 29 2.2.1 G l a c i a l Lake Thompson 30 2.2.2 D e g l a c i a t i o n of the F r a s e r -Thompson D i v i d e 34 3 TOE FRASER VALLEY - DEVELOPMENT OF CONTEMPORARY LANDFORMS 3 9 3.1 I n t r o d u c t i o n 3 9 3.2 The F r a s e r V a l l e y between Texas Creek and the B r i d g e R i v e r ^5 3.2.1 The L i l l o o e t S i l t 4 5 3.2.2 The Seton D e l t a 5 1 v i CHAPTER PAGE 3.2.3' The F o u n t a i n D e l t a 54 3.2.4 The R i v e r l a n d s Kame T e r r a c e . . . . 60 3.2.5 Bedrock Gorges a l o n g the F r a s e r . . 61 3.2.6 R i v e r T e r r a c e s 67 3.3 The F r a s e r V a l l e y between the B r i d g e R i v e r and P a v i l i o n . . . 70 3.3.1 S t r a t i g r a p h y of G r a v e l s beneath the Main Bench . 74 3.3.2 U n i t 1 - The Pebble T i l l 83 3.3.3 The F o u n t a i n Kame 91 3.3.4 Other G l a c i a l D e p o s i t s 92 3.3.5 A g g r a d a t i o n a l G r a v e l s above the Main Bench 93 3.3.6 The S t r u c t u r e of the P l e i s t o c e n e Depos i t s 94 3.3.7 P o s t - G l a c i a l D e p o s i t i o n 97 3.4 The F r a s e r V a l l e y between Texas Creek and L y t t o n 98 3.4.1 G l a c i a l T i l l . . 100 3.4.2 I c e - M a r g i n a l F e a t u r e s 102 3.4.3 A g g r a d a t i o n a l G r a v e l s beneath the Main Bench 105 3.5 Chronology f o r the F r a s e r V a l l e y between L y t t o n and P a v i l i o n 108 4 THOMPSON VALLEY - DEVELOPMENT OF CONTEMPORARY LANDFORMS 121 4.1 I n t r o d u c t i o n 121 4.2 L a c u s t r i n e D e p o s i t i o n . 124 4.2.1 The Spences B r i d g e S i l t 124 4.2.2 D e l t a i c G r a v e l s 142 4.2.3 Other L a c u s t r i n e D e p o s i t i o n . . . 149 4.3 Events F o l l o w i n g the Drainage of Lake Thompson 151 4.3.1 The E s t a b l i s h m e n t of S o u t h e r l y Drainage 151 4.3.2 Formation of R i v e r T e r r a c e s . . . 155 4.4 Chronology f o r the Thompson V a l l e y between A s h c r o f t and Spences B r i d g e 160 5 THE "BONAPARTE VALLEY - DEVELOPMENT OF CONTEMPORARY LANDFORMS . . 162 5.1 I n t r o d u c t i o n 162 5.2 G l a c i a l D e p o s i t i o n 164 5.3 F l u v i o - G l a c i a l D e p o s i t i o n 166 5.4 P o s t - G l a c i a l D e p o s i t i o n 168 v i i CHAPTER PAGE 6 THE KAMLOOPS AREA - DEVELOPMENT OF CONTEMPORARY LANDFORMS 171 6.1 I n t r o d u c t i o n 171 6.2 P l e i s t o c e n e H i s t o r y 172 6.3 D e g l a c i a t i o n 174 6.4 P o s t - G l a c i a l H i s t o r y 178 7 THE SIMILKAMEEN VALLEY - DEVELOPMENT OF CONTEMPORARY LANDFORMS 180 7.1 I n t r o d u c t i o n 180 7.2 G l a c i a l D e p o s i t s 183 7.2.1 L a t e r a l T i l l Sheet 183 7.2.2 B a s a l T i l l . 187 7.3 F l u v i o - G l a c i a l and G l a c i o - L a c u s t r i n e F e a t u r e s 190 7.3.1 The Similkameen-Okanagan D i v i d e . . 190 7.3.2 F e a t u r e s of the V a l l e y W a l l s . . . 191 7.3.3 The Similkameen S p i l l w a y 194 7.4 P o s t - G l a c i a l Development . . 194 REFERENCES - PART ONE ' 198 PART TWO. ALLUVIAL FANS IN BRITISH COLUMBIA . 202 8 INTRODUCTION TO PART TWO 203 8.1 8.1.1 D e f i n i t i o n of A l l u v i a l Fans. . . . 203 8.1.2 P r e v i o u s Work on P a r a - G l a c i a l A l l u v i a l Fans 205 8.2 G e n e r a l Hypotheses R e g a r d i n g Causes and C o n t r o l s of Fan F o r m a t i o n 206 8.2.1 E v o l u t i o n a r y Hypotheses 207 8.2.2 E q u i l i b r i u m Hypotheses 209 8.2.3 C l i m a t i c Hypotheses 211 8.3 8.3.1 P a r a - G l a c i a l A l l u v i a l Fans . . . . 214 8.3.2 A Comparison of the P a r a - G l a c i a l and A r i d A l l u v i a l Fans 218 8.4 S t a t i s t i c a l A n a l y s i s of Fan Morphometry . 219 8.4.1 D i s t r i b u t i o n of P a r t i c l e S i z e s . . 219 8.4.2 R e l a t i o n s h i p s between Fan and B a s i n 222 8.4.3 The Morphometry of P a r a - G l a c i a l A l l u v i a l Fans 226 9 THE COMMENCEMENT OF FAN AGGRADATION 229 9.1 9.1.1 The I d e n t i f i c a t i o n of Fan G r a v e l s . 229 9.2 Geometry of B a s a l Fan C o n t a c t s -T h e o r e t i c a l C o n s i d e r a t i o n s 230 9.3 Geometry of B a s a l Fan C o n t a c t s -O b s e r v a t i o n s and I n t e r p r e t a t i o n . . . . 235 v i i i CHAPTER PAGE 9.3.1 F r a s e r V a l l e y 235 9.3.2 Thompson V a l l e y 243 9.3.3 Bonaparte V a l l e y 244 9.3.4 Kamloops Area 245 9.3.5 Similkameen V a l l e y 246 9.3.6 ' Summary 249 10 THE CONSTRUCTION OF THE FANS 250 10.1 S t r a t i g r a p h y and S t r u c t u r e of the Fans. . 250 10.1.1 I n t r o d u c t i o n 250 10.1.2 F r a s e r V a l l e y 252 10.1.2 Thompson V a l l e y 266 10.1.4 Bonaparte V a l l e y 278 10.1.5 Kamloops Area 284 10.1.6 Similkameen V a l l e y .287 10.2 P r o c e s s e s of D e p o s i t i o n 291 10.2.1 The Mudflow P r o c e s s 291 10.2.2 Mudflow i n B r i t i s h Columbia Fans . 295 10.2.3 The F l u v i a l P r o c e s s - D e p o s i t i o n by Streams 304 10.2.4 F a c t o r s C o n t r o l l i n g D e p o s i t i o n upon Fans - Summ'ary ,. 311 10.3 The D u r a t i o n of Fan D e p o s i t i o n - A b s o l u t e D a t i n g 313 11 THE END OF FAN DEPOSITION AND POST-DEPOSITIONAL MODIFICATION 325 11.1 Recent D e p o s i t i o n upon Fans 325 11.1.1 D e p o s i t i o n by R o c k f a l l 325 11.1.2 D e p o s i t i o n from T a l u s S lopes . . . 328 11.1.3 D e p o s i t i o n below the I n t e r s e c t i o n P o i n t 329 11.1.4 D e p o s i t i o n of S c a t t e r e d S u r f a c e M a t e r i a l 332 11.1.5 F l u v i a l D e p o s i t i o n 333 11.1.6 Summary 333 11.2 The P r e s e n t C o n d i t i o n of Fans 334 11.2.1 R e l i c t Fans 335 .11.2.2 D i s s e c t e d Fans 337 11.2.3 Summary 363 11.3 Other Forms of P o s t - D e p o s i t i o n a l M o d i f i c a t i o n 364 11.3.1 T e l e s c o p e S t r u c t u r e Fans 364 11.3.2 Other M u l t i - L e v e l Fans 367 CHAPTER PAGE 12 BASIN MORPHOMETRY AND FAN GRADIENTS 375 12.1 12.1.1 Choice of Morphometric V a r i a b l e s f o r Fans 3 7 5 12.1.2 Choice of Morphometric V a r i a b l e s f o r Fan B a s i n s 377 12.2 12.2.1 R e s u l t s of C o r r e l a t i o n A n a l y s i s . . 378 12.2.2 Areas L a c k i n g S i g n i f i c a n t C o r r e l a t i o n s 381 12.3 12.3.1 R e s u l t s of R e g r e s s i o n A n a l y s e s . . 389 12.4 The R e l a t i o n s h i p between Fan G r a d i e n t and B a s i n P a r a m e t e r s : D i s c u s s i o n 1 + 0 0 12.4.1 F a c t o r s C o n t r o l l i n g F l u v i a l D e p o s i t i o n 4 0 0 12.4.2 F a c t o r s C o n t r o l l i n g Mudflow D e p o s i t i o n 4 0 5 12.5 The V a r i a t i o n of Fan - B a s i n R e l a t i o n s h i p s between the Study Areas 12.6 The D i s t i n c t i o n of P a r a - G l a c i a l Fans . . . 4 1 3 • 12.7 Summary 1 + 1 6 CONCLUSION 4 1 7 REFERENCES - PART TWO 422 APPENDIX #1 GRAIN-SIZE ANALYSES 426 APPENDIX #2 TABLE OF ORIGINAL DATA - FAN GRADIENTS AND BASIN MORPHOMETRIC VARIABLES 432 X LIST OF TABLES PAGE 1.1 P r e - P l e i s t o c e n e events i n f l u e n c i n g the development of landforms 9 1.2 G l a c i a l sequence i n Southwestern B r i t i s h Columbia and N o r t h w e s t e r n Washington . . . . 14 1.3 Radio-Carbon dates from I n t e r i o r B r i t i s h Columbia . . . . 18 3.1 Chronology f o r the F r a s e r V a l l e y 109 4.1 The T.singkahtle l a c u s t r i n e sequence 127 4.2 The Twaal d e l t a i c sequence 145 9.1 T h e o r e t i c a l c o n d i t i o n s of a g g r a d a t i o n 233 10.1 O r i g i n of mudflow m a t e r i a l 298 10.2 V o l c a n i c ash i n a l l u v i a l f a n s 314 11.1 Changes i n g r a d i e n t a c r o s s f a n apexes 338 11.2 Fan d i s s e c t i o n i n the F r a s e r V a l l e y 344 11.3 D i s s e c t i o n of Kamloops fans - depths of d i s s e c t i o n g u l l i e s 360 11.4 F r a s e r V a l l e y m u l t i - l e v e l f a n s 368 12.1 Product-moment c o r r e l a t i o n c o e f f i c i e n t s f o r f a n g r a d i e n t and v a r i o u s b a s i n morphometric v a r i a b l e s 379 12.2 R e s u l t s of r e g r e s s i o n a n a l y s e s of f a n g r a d i e n t a g a i n s t v a r i o u s b a s i n morphometric v a r i a b l e s 390 12.3 S i g n i f i c a n t d i f f e r e n c e s between s l o p e and i n t e r c e p t v a l u e s f o r r e g r e s s i o n l i n e s . . . . 392 x i LIST OF FIGURES PAGE 1.1 Southwestern B r i t i s h C o l u m b i a : . p h y s i o g r a p h i c r e g i o n s and l o c a t i o n of study areas 3 2.1 G l a c i a l Lake Thompson: l o c a t i o n map 32 2.2 P o s s i b l e g l a c i a l s p i l l w a y c o l s upon the F r a s e r -Thompson d i v i d e 37 3.1 F r a s e r V a l l e y : l o c a t i o n map 40 3.2 F r a s e r V a l l e y : bedrock geology 41 3.3 L i l l o o e t a r e a : landforms 46 3.4 The L i l l o o e t s i l t : c o n t o r t e d s i l t o v e r l a i n by c o b b l e g r a v e l 48 3.5 The L i l l o o e t s i l t : slump b a l l s i n Seton D e l t a . . . 48 3.6 The L i l l o o e t s i l t : c h a n n e l i n f i l l i n g 52 3.7 Seton D e l t a : f o r e s e t s of s i l t and f i n e sand . . . 52 3.8 S e c t i o n s w i t h i n the F o u n t a i n D e l t a 55< 3.9 F o u n t a i n D e l t a : i n f i l l e d k e t t l e h o l e 58 3.10 F o u n t a i n D e l t a : l a r g e - s c a l e c r o s s - b e d d i n g . . . . 58 3.11 F o u n t a i n D e l t a : r e l a t i v e l y f i n e f o r e s e t g r a v e l s above " c h a o t i c " g r a v e l s 59 3.12 F o u n t a i n D e l t a : v a r i a b l e c h a r a c t e r i s t i c s 59 3.13 Exposure beneath n o r t h e r n end of Jones bench . . . 66 3.14 F r a s e r V a l l e y : l o n g i t u d i n a l p r o f i l e ' 68 3.15 P a v i l i o n Creek t o B r i d g e R i v e r : l a n d f o r m s . . . . 71 3.16 F r a s e r V a l l e y near P a v i l i o n : s t r a t i g r a p h y beneath main bench 75 3.17 F r a s e r V a l l e y between P a v i l i o n and Lee Creeks. . . 80 3.18 F r a s e r V a l l e y near P a v i l i o n : c o a r s e g r a v e l bed . . 81 3.19 F r a s e r V a l l e y near P a v i l i o n : the Pebble T i l l . . . .85 3.20 F r a s e r V a l l e y near P a v i l i o n : cross-bedded g r a v e l . 81 x i i PAGE 3.21 F r a s e r V a l l e y near P a v i l i o n : g r a v e l s beneath main bench 95 3.22 C r o s s - s e c t i o n of F r a s e r V a l l e y near P a v i l i o n showing i n f e r r e d r e l a t i o n s h i p between d e p o s i t s 96 3.23. S e c t i o n s at 1 , 250 f e e t near F o u n t a i n showing i n t e r b e d d e d f l u v i a l and e a r t h f l o w m a t e r i a l . . 99 3.24 I n k o i k o Fan, s o u t h e r n l i m b : s i l t y t i l l 101 3.25 S p i n t l u m Creek exposure: t i l l r e s t i n g upon f l u v i o - g l a c i a l m a t e r i a l 103 3.26 The F r a s e r V a l l e y d u r i n g p o s t - g l a c i a l a g g r a d a t i o n : , v i e w downstream and c r o s s -s e c t i o n . . . 107 3.27 The F r a s e r V a l l e y d u r i n g p o s t - g l a c i a l a g g r a d a t i o n : view westwards and l o n g i -t u d i n a l s e c t i o n 107 3.28 Stage One I l l 3.29. Stage Two I l l 3.30 Stage Three 114 3.31 . Stage Four 114 3.32 Stage F i v e 119 3.33 Stage S i x 119 4.1 Thompson V a l l e y l o c a t i o n map 122 4.2 Thompson V a l l e y : bedrock geology 123 4.3 Thompson V a l l e y : benches, t e r r a c e s and a l l u v i a l f ans 125 4.4 Thompson V a l l e y : T s i n g k a h t l e l a c u s t r i n e sequence 129 4.5 G e n e r a l view of h i l l s i d e a d j a c e n t t o T s i n g k a h t l e exposure 130 4.6 P r o f i l e of e a s t e r n s i d e of Thompson V a l l e y showing the i n f e r r e d r e l a t i o n s h i p of s i l t and g r a v e l 132 x i i i PAGE 4.7 T s i n k a h t l e exposure: laminae and load-and— flame s t r u c t u r e s 133 4.8 T s i n g k a h t l e exposure: i n t e r l a m i n a t e d s i l t and f i n e sand 135 4.9 T s i n g k a h t l e exposure: i n t e r l a m i n a t e d s i l t and f i n d sand 135 4.10 T s i n g k a h t l e exposure: c u r r e n t r i p p l e marks . .. 136 4.11 T s i n g k a h t l e exposure: slump b a l l s w i t h i n the l a c u s t r i n e s i l t 136 4.12 T s i n g k a h t l e exposure: l o a d s t r u c t u r e s a l o n g c o n t a c t between mudflow g r a v e l and c o a r s e sand " . 139 4.13 G e n e r a l i z e d sequence of u n c o n s o l i d a t e d d e p o s i t s i n the Thompson V a l l e y 140 4.14 Thompson V a l l e y near Basque: sand w i t h c u r r e n t r i p p l e marks 139 4.15 Thompson V a l l e y : l o n g i t u d i n a l p r o f i l e 143 4.16 Twaal d e l t a i c sequence 147 4.17 Twaal d e l t a i c sequence: c o n t o r t i o n 150 4.18 Twaal d e l t a i c sequence: f l o w s t r u c t u r e . . . . 150 4.19 Exposure a t mouth of Pimainus Creek 152 4.20 View northwards near Basque: e r o s i o n a l t e r r a c e s 156 4.21 Thompson V a l l e y : topography and u n c o n s o l i d a t e d d e p o s i t s . . 157 4.22 Exposure a t mouth of Oregon Jack Creek showing l a c u s t r i n e s i l t o v e r l y i n g t e r r a c e g r a v e l . . 156 • 5.1 . Bonaparte V a l l e y : l andforms 163 5.2 Morphology o f the e a s t e r n s i d e of the Bonaparte V a l l e y 165 5.3 Morphology of the w e s t e r n s i d e of the Bonaparte V a l l e y 165 x i v PAGE 5.4 G e n e r a l view northwards a l o n g the Bonaparte V a l l e y : kame t e r r a c e s and a l l u v i a l f a n s . . . 167 6.1 Kamloops Ar e a : s u r f i c i a l geology and l a n d f o r m s . 173 6.2 Scheidam Creek v a l l e y : g l a c i o - l a c u s t r i n e sediments 176 6.3 Morphology of the South Thompson V a l l e y : the w h i t e s i l t bench and s c a r p 176 7.1 Similkameen V a l l e y : l andforms 181 7.2 Similkameen V a l l e y : bedrock geology 182 7.3 L a t e r a l t i l l s h e e t : b a s a l l a y e r 185 7.4 L a t e r a l t i l l s h e e t : upper s e c t i o n of t i l l "hoodoo" 185 7.5 Morphology of l a t e r a l t i l l sheet r e s u l t i n g from e r o s i o n : "hoodoos" and p a r a l l e l g u l l i e s . . 186 7.6 O r i g i n a l morphology of l a t e r a l t i l l s h e e t : l a t -e r a l r i d g e and c h a n n e l 186 7.7 Similkameen V a l l e y : s t r a t i g r a p h y from w e l l r e c o r d s 189 7.8 Kame t e r r a c e s and d e l t a s between Robert and Susap Creeks 193 7.9 S m a l l f l u v i o - g l a c i a l t e r r a c e s a s s o c i a t e d w i t h s m a l l creek south of B l i n d Creek 193 7.10 Mazama ash i n f o o t of t a l u s s l o p e near Similkameen s t a t i o n 197 9.1 T h e o r e t i c a l s t r u c t u r e s of a l l u v i a l f a n s . . . . 234 9.2 G e n e r a l i z e d s t r a t i g r a p h y a t toe of M c G i l l i v r a y Creek f a n . 238 9.3 F r a s e r V a l l e y near K e a t l e y Creek: a l l u v i a l f a n s v e n e e r i n g main bench 241 9.4 Kamloops Area: r i v e r b a n k exposure between f a n #1 and f a n #2 247 XV PAGE 9.5 Similkameen f a n #4: r i v e r b a n k exposure . . . . 241 10.1 G e n e r a l i z e d s t r a t i g r a p h y of s o u t h e r n l i m b of C i n q u e f o i l Creek f a n 253 10.2 R i p p l e mark sequence 255 10.3 M c G i l l i v r a y f a n : s o u t h e r n l i m b viewed from the e a s t 258 10.4 M c G i l l i v r a y f a n : s o u t h e r n l i m b viewed from the west 258 10.5 M c G i l l i v r a y f a n : s o u t h e r n l i m b w i t h f o r e s e t bedding . 259 10.6 I ce Cave f a n : mudflow beds o v e r l y i n g s i l t and sand , 259 10.7 S t r a t i g r a p h i c s e c t i o n i n Ice Cave f a n . ; . . . . 261 10.8 L a l u w i s s e n f a n g r a v e l 263 10.9 L a l u w i s s e n f a n g r a v e l 2 5 3 10.10 S t r u c t u r e of the toe of f a n near L a l u w i s s e n Creek 265 10.11 Spatsum f a n s : exposure a t the toe of Fan #15 (C o l d s t r e a m Creek) 268 10.12 Spatsum f a n s : exposure at the toe of f a n #14 . 268 10.13 T h e o r e t i c a l p r o f i l e through Spatsum-type f a n . 273 10.14 Spatsum f a n s : exposure at the toe of f a n #11 . 277 10.15 S t r a t i g r a p h i c s e c t i o n at the toe of Bonaparte f a n #1 280 10.16 Bonaparte f a n #1: exposure at toe 281 10.17 S t r a t i g r a p h i c s e c t i o n at c o a l e s c i n g t o e s of Bonaparte fans #8 and #9 283 10.18 Similkameen f a n #5: exposure down o n e - t h i r d of f a n from apex 288 10.19 Similkameen f a n #5: s u r f a c e of upper p a r t of f a n w i t h l o b a t e b o u l d e r p a t t e r n 288 X V I PAGE 10.20 Similkameen f a n #4: p r o f i l e view northwards . 303 10.21 Mazama v o l c a n i c ash w i t h i n s m a l l f a n south of Twaal Creek 317 10.22 Mazama v o l c a n i c ash exposed by road c u t t i n g t h r ough Bonaparte f a n 317 10.23 Mazama (?) v o l c a n i c ash band i n t r u n c a t e d s o u t h e r n l i m b of N i c o l a t e l e s c o p e s t r u c t u r e f a n 319 10.24 Mazama ash i n toe of s m a l l f a n near Cawston, Similkameen V a l l e y . 320 10.25 Mazama and S t . Helen's ash bands i n w a l l of J u n i p e r Creek d i s s e c t i o n g u l l y 319 11.1 Fan a g g r a d a t i o n due t o r o c k f a l l 326 11.2 Kamloops f a n #1, t o e : view downfan a l o n g g r a v e l l o b e 331 11.3 Thompson V a l l e y near Oregon Jack Creek: s m a l l f a n w i t h s c a t t e r e d , r e c e n t l y d e p o s i t e d p e b b l e s 331 11.4 B a s e - l e v e l d i s s e c t i o n 340 11.5 B a s e - l e v e l d i s s e c t i o n : t y p i c a l f a n p r o f i l e s . 342 11.6 P r o p o r t i o n a l depths of d i s s e c t i o n g u l l i e s . . 343 11.7 The development of d i s s e c t i o n g u l l i e s . . . . 349 11.8 F r a s e r V a l l e y : view upstream of d i s s e c t i o n g u l l y of Texas Creek 352 11.9 D i s s e c t i o n at toe of fan due t o u n d e r c u t t i n g by major r i v e r 354 11.10 Fan morphology t y p i c a l of Kamloops Area. . . 355 11.11 D i s s e c t i o n due to fan-head t r e n c h i n g . . . . 356 11.12 Kamloops f a n #12: morphology of embayment. . 359 11.13 Kamloops f a n #2: upper p a r t of b a s i n . . . . 352 xv i i PAGE 11.14 T y p i c a l morphology of " t e l e s c o p e s t r u c t u r e " f a n , F r a s e r V a l l e y 366 11.15 N i c o l a Fan: t e l e s c o p e s t r u c t u r e 359 11.16 T y p i c a l morphology of m u l t i - l e v e l f a n , Thompson V a l l e y 369 11.17 Development of a m u l t i - l e v e l f a n 371 11.18 C i n q u e f o i l Creek f a n 373 12.1 Graphs ( l o g a r i t h m i c ) t o show morphometric r e l a t i o n s h i p s f o r the Kamloops area 383 12.2 S c a t t e r diagrams ( l o g a r i t h m i c ) of f a n g r a d i e n t a g a i n s t b a s i n r e l a t i v e r e l i e f number f o r F r a s e r E a s t 387 12.3 Graphs ( l o g a r i t h m i c ) t o show the r e l a t i o n s h i p between f a n g r a d i e n t and r e l a t i v e r e l i e f number 393 12.4 Comparison of r e g r e s s i o n l i n e s f o r the r e l a t i o n s h i p between f a n g r a d i e n t and r e l a t i v e r e l i e f number 394 12.5 Graphs ( l o g a r i t h m i c ) t o show the r e l a t i o n s h i p between f a n g r a d i e n t and b a s i n h e i g h t . . . 395 .12.6 Graphs ( l o g a r i t h m i c ) t o show the r e l a t i o n s h i p between f a n g r a d i e n t and b a s i n area 396 12.7 Comparison of r e g r e s s i o n l i n e s f o r the r e l a t i o n -s h i p between f a n g r a d i e n t and b a s i n a rea . . 397 12.8 Graphs ( l o g a r i t h m i c ) to show the r e l a t i o n s h i p between f a n g r a d i e n t and b a s i n s l o p e . . . . 398 12.9 I n t e r r e l a t i o n s h i p s between f a n and b a s i n parameters ^06 ACKNOWLEDGEMENTS I would l i k e t o than k , f i r s t , Dr. Mark A. M e l t o n and Dr. W. H. Mathews.' Dr. Me l t o n s u p e r v i s e d the t h e s i s and gave u s e f u l h e l p and a d v i c e d u r i n g a l l s t a g e s of i t s p r o d u c t i o n ; he a l s o made an attempt to h e l p i n the f i e l d . Dr. Mathews p r o v i d e d v a l u a b l e d i s c u s s i o n , e s p e c i a l l y c o n c e r n i n g a s p e c t s of the Quaternary h i s t o r y and gave g e n e r a l encouragement d u r i n g the w r i t i n g . Thanks are a l s o due to Dr. J . R. Mackay and Dr. J . K. Sta g e r who re v i e w e d the f i n a l d r a f t . Dr. S t a g e r , a c t e d as a d m i n i s -t r a t i v e s u p e r v i s o r a f t e r Dr. Melton's "departure from U. B. C. I n v a l u a b l e a s s i s t a n c e and d i s c u s s i o n d u r i n g the course of f i e l d work was v o l u n t e e r e d by Miss L e s l e y J . And e r t o n ; Messrs. G. C o o t s , J . P. Crowley, T. Day and M. K. Woo a l s o v o l u n t e e r e d o c c a s i o n a l a s s i s t a n c e i n the f i e l d . I would l i k e t o thank the f o l l o w i n g f o r a d v i c e and i n f o r m a t i o n r e g a r d i n g s p e c i f i c problems: Dr. L. R. K i t t l e m a n of the U n i v e r s i t y of Oregon and Dr. R. J . F u l t o n of the G e o l o g i c a l Survey of Canada f o r a d v i c e r e g a r d i n g the i d e n t i f i c a t i o n of v o l c a n i c a s h ; Dr. J . C. Foweraker of the B r i t i s h Columbia Department of Lands, F o r e s t s and Water Resources f o r data from w e l l l o g s ; Mr. J . P. Crowley f o r use of c e r t a i n morphometric d a t a ; and Dr. J . de V r i e s of the Department of S o i l S c i e n c e , U.B.C. f o r making a v a i l a b l e l a b o r a t o r y f a c i l i t i e s f o r g r a i n - s i z e a n a l y s e s . The f i e l d work was c a r r i e d out w i t h the a i d of U n i v e r s i t y of B r i t i s h Columbia Research Grants o b t a i n e d by Dr. J . K. St a g e r i n 1964 and by Dr. M. A. Me l t o n i n 1965 and 1966. INTRODUCTION A l l u v i a l fans are common f e a t u r e s of the i n t e r i o r v a l l e y s of B r i t i s h Columbia. They occur i n a s s o c i a t i o n w i t h a g g r a d a t i o n a l s u r f a c e s and r i v e r t e r r a c e s at p o i n t s where t r i b u t a r i e s e n t e r major v a l l e y s . Most of these fans are r e l i c t f e a t u r e s , not c u r r e n t l y undergoing a g g r a d a t i o n b u t , i n f a c t , b e i n g d e s t r o y e d . They were b u i l t by p r o c e s s e s t h a t were most a c t i v e d u r i n g some p a s t , but p o s t - g l a c i a l i n t e r v a l of ti m e . Most p r e v i o u s work d e a l i n g w i t h a l l u v i a l f a n s i n North America has been done w i t h i n the c o n t e x t of the a r i d l a n d s c a p e . T h i s t h e s i s attempts to d i s t i n g u i s h between the a r i d - r e g i o n fans and those of B r i t i s h Columbia. The l a t t e r c l o s e l y p o s t - d a t e a g l a c i a t i o n and r e s u l t from i t s e f f e c t s ; they were formed d u r i n g the g e n e r a l and widesp r e a d phase of v a l l e y a g g r a d a t i o n t h a t commonly f o l l o w s g l a c i a t i o n of a mountainous r e g i o n . S p e c i f i c areas were chosen w i t h i n s o u t h - c e n t r a l B r i t i s h Columbia f o r the i n v e s t i g a t i o n of a l l u v i a l f a n s . They were s e l e c t e d w i t h r e g a r d t o the o c c u r r e n c e of a group of s i m i l a r f a n s , r a t h e r than i s o l a t e d examples, i n o r d e r t h a t s t a t i s t i c a l comparisons of f a n c h a r a c t e r i s t i c s might be made among the d i f f e r e n t a r e a s . A l s o , the r e g i o n s d i f f e r w i t h r e g a r d t o the s u i t e of p o s t - g l a c i a l landforms w i t h which the fans are a s s o c i a t e d . 2 The f i v e areas s e l e c t e d were: (1) the 60-mile s t r e t c h of the F r a s e r V a l l e y between P a v i l i o n and L y t t o n ; (2) the Thompson V a l l e y between Spences B r i d g e and a p o i n t 20 m i l e s f a r t h e r n o r t h near A s h c r o f t ; (3) the Bonaparte V a l l e y from 2 m i l e s n o r t h of.Cache Creek to C a r q u i l l e ; (4) the 5-miles s t r e t c h of the South Thompson V a l l e y i m m e d i a t e l y t o the east of Kamloops ( r e f e r r e d to as the "Kamloops Area") and (5) the Similkameen V a l l e y over a d i s t a n c e of 10 m i l e s between Keremeos and the I n t e r n a t i o n a l Boundary. The l o c a t i o n of these study areas i s g i v e n i n F i g . 1.1. With the e x c e p t i o n of the F r a s e r V a l l e y , a l l t h e s e areas l i e w i t h i n the I n t e r i o r P l a t e a u of B r i t i s h Columbia. The l o c a l r e l i e f v a r i e s from a minimum of 2,000' i n the Kamloops Area to a maximum of s l i g h t l y more than 7,000' i n the Similkameen V a l l e y . The F r a s e r V a l l e y l i e s a l o n g the boundary l i n e between the I n t e r i o r P l a t e a u and the Coast M o u n t a i n s , a s i t u a t i o n which g i v e s r i s e to some i n t e r e s t i n g c o n t r a s t s between the east and west banks of the F r a s e r R i v e r . Fans were s e l e c t e d f o r study w i t h i n these r e g i o n s by v i r t u e of t h e i r l a r g e s i z e and a c c e s s i b i l i t y . Not a l l fans were i n v e s t i g a t e d i n every a r e a ; f o r example, i n the Thompson V a l l e y a major group of fans r e s t i n g upon d e g r a d a t i o n a l r i v e r t e r r a c e s were s t u d i e d i n p r e f e r e n c e to e a r l i e r fans of which o n l y s m a l l remnants remain. S i n c e the c o n s t r u c t i o n of fans i n B r i t i s h Columbia was FIGURE I. SOUTHWESTERN BRITISH COLUMBIA -PHYSIOGRAPHIC REGIONS AND LOCATION OF STUDY AREAS (3) ,BONAPAR "vLVALLEY C O A M 0 U hhT A (Paci f ic Ranges) FRASER ^ i v ^ , ' - . tfJTHOMP: ( I ) VALLEY VALLEY |!%& - study areas (numbered) -boundary of major physiographic regions -subdivisions of major physiographic regions Sea le 2 0 10 o 2 0 40 . miles Physiographic Regions after S S. Holland, 1964 4 not an i s o l a t e d phenomenon but c l o s e l y r e l a t e d i n both space and time to o t h e r geomorphic e v e n t s , a c o n s i d e r a b l e know-ledge of o t h e r p r o c e s s e s a c t i v e at the same time as f a n c o n s t r u c t i o n i s n e c e s s a r y . Except f o r the case of the Kamloops A r e a , t h e r e has been no p r e v i o u s work s p e c i f i c a l l y d e a l i n g w i t h the Quaternary h i s t o r y of the study a r e a s . C o n s e q u e n t l y , a g r e a t d e a l of f i e l d work and subsequent a n a l y s i s was concerned w i t h d e r i v i n g t h i s background i n f o r m -a t i o n . A c c o r d i n g l y , t h i s t h e s i s i s d i v i d e d i n t o two p a r t s : P a r t One (C h a p t e r s I t o V I I ) d e a l s w i t h the i n t e r p r e t a t i o n of a l l Quaternary d e p o s i t s and landforms i n o r d e r t o e s t -a b l i s h a c h r o n o l o g y f o r each r e g i o n ; P a r t Two (C h a p t e r s V I I I t o X I I ) d e a l s s p e c i f i c a l l y w i t h problems r e l a t e d to the a l l u v i a l fans and u t i l i z e s much i n f o r m a t i o n from P a r t One. PART ONE THE PHYSIOGRAPHIC HISTORY OF SELECTED REGIONS IN SOUTH-CENTRAL BRITISH COLUMBIA CHAPTER I THE PHYSIOGRAPHIC HISTORY OF SOUTH-CENTRAL BRITISH COLUMBIA A GENERAL OUTLINE L i t t l e i n f o r m a t i o n i s a v a i l a b l e r e g a r d i n g the development of landforms i n B r i t i s h Columbia p r i o r t o the m e l t i n g of the P l e i s t o c e n e i c e c o v e r . A b r i e f survey of p u b l i s h e d m a t e r i a l i s p r e s e n t e d below i n o r d e r t o g i v e a g e n e r a l p i c t u r e o f the e a r l y stages of e v o l u t i o n of the p r e s e n t l a n d s c a p e . The unique c h a r a c t e r of i n d i v i d u a l v a l l e y s appears to be l a r g e l y the r e s u l t of the p a t t e r n of d e g l a c i a t i o n and p o s t - g l a c i a l development which i s d i s c u s s e d i n the c h a p t e r s t h a t f o l l o w . 1.1 Development of Landforms b e f o r e the P l e i s t o c e n e  G l a c i a t i o n I t i s p o s s i b l e t h a t the p r e s e n t morphology of so u t h -c e n t r a l B r i t i s h Columbia was i n i t i a t e d d u r i n g C r e t a c e o u s ti m e . D u r i n g the mid-Mesozoic orogeny the e u g e o s y n c l i n a l r o c k s of the c o a s t a l and c e n t r a l areas of B r i t i s h Columbia were u p l i f t e d , deformed and i n t r u d e d by g r a n i t e p l u t o n s a l o n g the a x i s of the p r e s e n t Coast Mountains. Both d u r i n g and a f t e r t h i s orogeny, l a t e - M e s o z o i c sediments ( c h i e f l y C r e t a c e o u s ) and v o l c a n i c s accumulated i n a s e r i e s of d i s -connected b a s i n s i n both i n t e r i o r and c o a s t a l a r e a s . They were d e r i v e d from the mountains where the g r a n i t i c c o res of b a t h o l i t h s became exposed as e r o s i o n p r o g r e s s e d ( D u f f e l l and 7 McTaggart, 1952; H o l l a n d , 1964; K i n g , 1966). Thus by l a t e -C r e t a c e o u s time t h e r e may have e x i s t e d an e r o s i o n a l l a n d s u r f a c e where the broad o u t l i n e s of the p r e s e n t p a t t e r n of topography and d r a i n a g e were r e p r e s e n t e d (see Table 1.1). The end of C r e t a c e o u s and the b e g i n n i n g of T e r t i a r y time was marked by f u r t h e r d i a s t r o p h i s m d u r i n g which the Coast Mountains were e l e v a t e d w i t h r e s p e c t to the I n t e r i o r P l a t e a u . Eocene sedimentary and v o l c a n i c r o c k s were d e p o s i t e d i n l a k e s and v a l l e y s , ( D u f f e l l and McTaggart, 1952; H o l l a n d , 1964; Lay, 1940; Uglow, 1922) and l a t e r , g e n t l y deformed, (Mathews, 1968). There f o l l o w e d a r e l a t i v e l y l o n g p e r i o d of s t a b i l i t y d u r i n g which an e r o s i o n a l s u r f a c e of low r e l i e f was formed. T h i s s u r f a c e t r u n c a t e d f o l d s i n the Eocene v o l c a n i c s (Kamloops Group) but sup p o r t e d undeformed l a t e - M i o c e n e and e a r l y - P l i o c e n e l a v a f l o w s and some f l a t - l y i n g s e dimentary r o c k s near P a v i l i o n i n the F r a s e r V a l l e y , ( H o l l a n d , 1964; Mathews and Rouse, 1963; Mathews 1964; T r e t t i n , 1960). T h i s Miocene e r o s i o n s u r f a c e i s the " l a t e - T e r t i a r y pene-p l a i n " , f i r s t r e c o g n i z e d and d e s c r i b e d by G. M. Dawson (1889). Today i t remains as the r o l l i n g u pland s u r f a c e of the I n t e r i o r P l a t e a u and i s thus the o l d e s t geomorphic element of the p r e s e n t topography (see Table 1.1). P l i o c e n e u p l i f t ( d a t e d as post e a r l y - P l i o c e n e by the i n c i s i o n of e a r l y - P l i o c e n e p l a t e a u b a s a l t s ; Mathews, 1964) f o l l o w e d the same p a t t e r n as p r e v i o u s u p l i f t s w i t h the g r e a t e s t amount of v e r t i c a l movement t a k i n g p l a c e a l o n g 8 the a x i s of the Coast Mountains (Dawson, 1889; D r y s d a l e , 1912; Mathews, 1968). The Okanagan Range was a l s o e l e v a t e d at t h i s time w h i l s t the p r e v i o u s l y e s t a b l i s h e d Similkameen R i v e r m a i n t a i n e d i t s course t r a n s v e r s e t o t h i s major s t r u c t u r e ; hanging t r i b u t a r y v a l l e y s near Hedley r e s u l t from t h i s u p l i f t r a t h e r than from g l a c i a l e f f e c t s ( C a m s e l l , 1910; R i c e , 1947). T r e n c h i n g a l o n g the major r i v e r s and t r i b u t a r i e s was i n i t i a t e d by the P l i o c e n e u p l i f t and r e s u l t e d i n the p a r t i a l d i s s e c t i o n of the upland s u r f a c e . S i n c e the e r o s i o n worked headwards a l o n g the s t r eams, the degree of d i s s e c t i o n of the p l a t e a u v a r i e s w i t h d i s t a n c e upstream (see Table 1.1). Thus at the end of the T e r t i a r y p e r i o d , the l a n d s u r f a c e was one of c o n s i d e r a b l e r e l i e f w i t h w e l l d e f i n e d v a l l e y , p l a t e a u and mountain a r e a s . These major elements of topography were not unduly a l t e r e d by the g l a c i a t i o n t h a t f o l l o w e d . 1.1.1 The P r e - G l a c i a l F r a s e r V a l l e y D u r i n g l a t e P l i o c e n e the c o n f i g u r a t i o n of t h i s v a l l e y may have been v e r y s i m i l a r to t h a t of the p r e s e n t bedrock s u r f a c e . The r i v e r o c c u p i e d the i n n e r gorge of a v a l l e y w i t h a "V i n V" c r o s s p r o f i l e t h a t r e s u l t e d from r e j u v e n a t i o n i n i t i a t e d by the P l i o c e n e u p l i f t . . T r e t t i n (1960) d e s c r i b e s an e r o s i o n s u r f a c e of m i d d l e - to l a t e - T e r t i a r y age t h a t r e p r e s e n t s a former f l o o r of the F r a s e r V a l l e y and extends from Glen F r a s e r (near P a v i l i o n ) t o B i g Bar Mountain. I t i s l i k e l y t h a t t h i s was TABLE 1.1 PRE-PLEISTOCENE EVENTS INFLUENCING THE DEVELOPMENT OF LANDFORMS P e r i o d Epoch Events and Landforms R e s u l t i n g QUATERNARY P l e i s t o c e n e G l a c i a t i o n T r e n c h i n g a l o n g major v a l l e y s and t r i b u t a r i e s P l i o c e n e U p l i f t - Coast Mountains and Okanagan Range e l e v a t e d r e l a t i v e to I n t e r i o r P l a t e a u TERTIARY Miocene E r o s i o n a l s u r f a c e o f . l o w r e l i e f 0 l i g o c e n e Eocene D e p o s i t i o n of sediments i n major v a l l e y s ; d e f o r m a t i o n of s t r a t i f i e d r o c k s . E r o s i o n a l o n g major v a l l e y s . - - U p l i f t - Coast Mountains e l e v a t e d r e l a t i v e t o I n t e r i o r P l a t e a u CRETACEOUS E r o s i o n a l s u r f a c e E r o s i o n and s e d i m e n t a t i o n i n b a s i n s JURASSIC Orogeny - i n t r u s i o n of Coast Mtn--b a t h o l i t h s 10 p a r t of the widespread Miocene e r o s i o n s u r f a c e . Remnants of t h i s s u r f a c e a l s o e x i s t over the C l e a r and Camelsfoot ranges a d j a c e n t to the P a v i l i o n - L i l l o o e t s e c t i o n of the F r a s e r V a l l e y . The upper p a r t s of the v a l l e y s t h a t d r a i n from these ranges are a l s o thought to date from the same t i m e , and t o have been o n l y s l i g h t l y m o d i f i e d by g l a c i a l a c t i o n . They are wide v a l l e y s w i t h g e n t l e s i d e s l o p e s and g e n t l e l o n g i t u d i n a l g r a d i e n t s . Dawson (1889) r e c o g n i z e d these "upland v a l l e y s " and gave L a l u w i s s e n and P a v i l i o n Creeks as examples. The break of s l o p e or " l i p " of t h e s e hanging v a l l e y s may be taken as an i n d i c a t i o n of the maximum, e l e v a t i o n of the main v a l l e y f l o o r b e f o r e the P l i o c e n e r e j u v e n a t i o n . T i f f i n , C i n q u e f o i l and Izman Creeks a l s o have w e l l d e f i n e d hanging v a l l e y s . The maximum p o s s i b l e amounts of d o w n c u t t i n g due to a com-b i n a t i o n of P l i o c e n e u p l i f t and g l a c i a l e r o s i o n are i n d i c a t e d i n the f o l l o w i n g t a b l e : V a l l e y E l e v a t i o n of Source Height above p r e s e n t l i p r i v e r bed  L a l u w i s s e n 3,450' Dawson (1889) 2,895' P a v i l i o n 2,300' Dawson ( " ) 1,520' T i f f i n 3,200' Ryder" 2,430' C i n q u e f o i l 2,000' Ryder 1,425' Izman 2,400' Ryder 1,870' I f the i n t e r p r e t a t i o n p l a c e d upon the s e landforms i s c o r r e c t , then the r i g h t hand column g i v e s some i n d i c a t i o n of the amount of v e r t i c a l e r o s i o n a c c o m p l i s h e d by the F r a s e r E l e v a t i o n s i n t e r p o l a t e d from map w i t h 100' C . I . ; a c c u r a t e to n e a r e s t 100'. 11 d u r i n g l a t e - P l i o c e n e and Quaternary t i m e . The c o n s i d e r -a b l e v a r i a t i o n i n these f i g u r e s may be a t t r i b u t e d t o l o c a l d i f f e r e n c e s of r a t e s of e r o s i o n c o n t r o l l i n g the exact p o s i t i o n of the k n i c k p o i n t s . Thus i t may be t e n t a t i v e l y c oncluded t h a t between 1,400' and 2,900' of d o w n c u t t i n g took p l a c e . These v a l u e s are of the same or d e r of magnitude as e s t i m a t e s g i v e n by H o l l a n d (1964) f o r s i m i l a r d o w n c u t t i n g i n the F r a s e r Canyon. Here t h e r e i s a convex break of s l o p e between the P l i o c e n e v a l l e y s i d e s and the younger gorge, the Canyon i t s e l f . The break i s between 1,500' and 2,000' above p r e s e n t r i v e r l e v e l . A s i m i l a r t o p o g r a p h i c break was i d e n t i f i e d by the w r i t e r on the e a s t e r n bank of the F r a s e r between P a v i l i o n and Gibbs Creeks at 2,700' above r i v e r l e v e l . From a l l of the above e v i d e n c e , an average v a l u e of about 2,000' may be taken f o r the l o w e r i n g of the f l o o r of the F r a s e r V a l l e y s i n c e the P l i o c e n e u p l i f t . 1. 2 The P l e i s t o c e n e G l a c i a t i o n The growth of the e a r l y P l e i s t o c e n e g l a c i e r s was c o n t r o l l e d by the c o n d i t i o n s of c l i m a t e and topography t h a t e x i s t e d at the c l o s e of P l i o c e n e t i m e . The mode of expan-s i o n of the i c e sheet has been summarized by F l i n t (1957) as f o l l o w s : 12 "There i s good e v i d e n c e t h a t the growth of the g r e a t c o a l e s c e n t i c e sheet i n B r i t i s h Columbia o c c u r r e d i n f o u r phases: (1) an a l p i n e phase, i n which s m a l l v a l l e y g l a c i e r s developed on the m o i s t Coast Ranges and to a l e s s e r e x t e n t on the d r i e r Rocky Mountains f a r t h e r e a s t ; (2) an i n t e n s e -a l p i n e phase, i n which growth and c o a l e s c e n c e of the v a l l e y g l a c i e r s l e d to the development of l o n g t r u n k v a l l e y g l a c i e r s i n major v a l l e y s ; (3) a m o u n t a i n - i c e - s h e e t phase, i n which f u r t h e r growth developed e x t e n s i v e piedmont g l a c i e r s a l o n g the f l a n k s of the mountains, e s p e c i a l l y the Coast-Ranges; t h i s i c e became so t h i c k t h a t i t v i r t u a l l y b u r i e d the Coast Ranges; and (4) a c o n t i n e n t a l -i c e - s h e e t phase, i n which, over the c o u n t r y between Coast Ranges and R o c k i e s , the piedmonts c o a l e s c e d i n t o an i c e sheet a t l e a s t 7,500' t h i c k over the v a l l e y s . T h i s i c e was c o n f i n e d between the i n c l o s i n g ranges and had l i t t l e movement. From t h i s c e n t r a l r e s e r v o i r , w i t h a domelike c e n t e r near l a t 53°, o u t l e t g l a c i e r s d i s c h a r g e d westwards through v a l l e y s t r a n s e c t i n g the Coast Ranges, e r o d i n g them i n t e n s e l y . " ( F l i n t , 1957 , p. 305. ) 1 E v i d e n c e to i n d i c a t e the o c c u r r e n c e of more than one g l a c i a l stage d u r i n g the P l e i s t o c e n e has been found over most of s o u t h e r n B r i t i s h Columbia. However, the amount of m a t e r i a l a v a i l a b l e at p r e s e n t i s s u f f i c i e n t to war-rant- o n l y t e n t a t i v e attempts at c o r r e l a t i o n between d i f f e r e n t r e g i o n s . 1.2.1 Sequence of G l a c i a t i o n s on the Coast The b e s t documented evidence of m u l t i p l e g l a c i a t i o n s comes from the F r a s e r and Puget l o w l a n d s . Here t h e r e i s s t r a t i g r a p h i c e v idence f o r f o u r major g l a c i a l s t a g e s and t h r e e i n t e r s t a d i a l s ( E a s t e r b r o o k , C r a n d e l l and L e o p o l d , From D a v i s and Mathews, 1944; Armstrong and T i p p e r , 1948; Armstrong and Brown., 1954. 13 1967 ) , see Table 1.2. E v i d e n c e of m u l t i p l e g l a c i a t i o n s from the Lower F r a s e r V a l l e y i s r e p o r t e d by Armstrong i n s e v e r a l pub-l i c a t i o n s ( A r m s t r o n g , 1956a, 1957, 1960a and i n H a l s t e a d , 1957). Here, s t r a t i g r a p h y i n d i c a t e s a t l e a s t t h r e e major g l a c i a l s t a g e s named Seymour, Semiamu and Vashon i n o r d e r of o c c u r r e n c e . One major i n t e r g l a c i a l , the Quadra, o c c u r r e d b e f o r e the Vashon, and p r o b a b l y f o l l o w e d the Semiamu stage ( A r m s t r o n g , 1960a, Armstrong, C r a n d e l l , E a s t e r b r o o k and N o b l e , 1965 ).' T h i s c o r r e s p o n d s t o the Olympia i n t e r g l a c i a l ( E a s t e r b r o o k , et_ a l . , 1967 ), see Table 1.2. A d e t a i l e d account of the most r e c e n t g l a c i a l s t a g e , the F r a s e r G l a c i a t i o n , i s g i v e n by Armstrong et_ a l . ( 1 9 6 5 ) , who c o r r e l a t e t h i s w i t h the " c l a s s i c a l W i s c o n s i n " ( F l i n t , 1963) of e a s t e r n N o r t h America. In the Puget-F r a s e r l o w l a n d the F r a s e r G l a c i a t i o n had s e v e r a l sub-s t a g e s , i n d i c a t e d i n Table 1.2. The Evans Creek Stade was a p e r i o d of a l p i n e g l a c i a -t i o n i n the mountains of w e s t e r n Washington, but d u r i n g t h i s time i c e from the Canadian mountains expanded beyond the a l p i n e stage to r e a c h the l o w l a n d s as a p r o l o n g a t i o n of the Vashon g l a c i a t i o n . C o n s e q u e n t l y , the F r a s e r G l a c i a t i o n over most of the Lower F r a s e r V a l l e y i s r e p r e s e n t e d by t h r e e s u b - s t a g e s , the Vashon Stade, the Everson I n t e r s t a d e (marked by the d e p o s i t i o n of the C a p i l a n o g r a v e l s ) and the Sumas Stade ( A r m s t r o n g , et_ a_l. , 1965 ). TABLE 1.2 GLACIAL SEQUENCE IN SOUTHWESTERN BRITISH COLUMBIA AND NORTHWESTERN WASHINGTON T IT D A T E „ , - I T -i j Lower F r a s e r , C o a s t a l Lowland __ , - y e a r V a l l e y J BP . FRASER GLACIATION Sumas Stade - -Sumas Stade ^ Q O Q Everson I n t e r s t a d e - - C a p i l a n o Seds. Vashon Stade -Vashon Stade Evans Creek Stade - _ „ n r. zb ,l)OU OLYMPIA INTERGLACIATION - -- -Quadra Beds 36,000 SALMON SPRINGS GLACIATION Semiamu G l a c i a t i o n (age unknown) PUYALLUP INTERGLACIATION Seymour G l a c i a t i o n (age unknown) STUCK GLACIATION Unnamed g l a c i a t i o n (age unknown) ALDERTON INTERGLACIATION ORTINGTON GLACIATION Dates are approximate due to the t r a n s g r e s s i v e n a t u r e of the g l a c i a l and i n t e r g l a c i a l s t a g e s . (Armstrong 1965a, 1957; H a l s t e a d 1957; Armstrong 1960a; Armstrong, C r a n d e l l , E a s t e r b r o o k and Noble 1965; E a s t e r b r o o k , C r a n d e l l and L e o p o l d 1967; INQUA Guidebook, 1965) 15 D u r i n g the Sumas Stade, a v a l l e y g l a c i e r o c c u p i e d the e a s t e r n p a r t of the F r a s e r l o w l a n d . T h i s p o s s i b l y r e p r e s e n t e d the l a s t readvance of the C o r d i l l e r a n g l a c i e r (Armstrong et a l . , 1965 ) . 1.2.2 Sequence of G l a c i a t i o n s I n l a n d •In the I n t e r i o r P l a t e a u , Dawson (1889 ) r e c o g n i s e d two g l a c i a l p e r i o d s s e p a r a t e d by an i n t e r g l a c i a l s t a g e . He was the f i r s t t o suggest t h a t t h e r e had f o r m e r l y e x i s t e d a " C o r d i l l e r a n G l a c i e r " which o r i g i n a t e d by the expansion of i c e from the mountains, but l a t e r developed a major a c c u m u l a t i o n zone to the east of the Coast Mountains. T h i s was l o c a t e d between one and two hundred m i l e s i n l a n d and between l a t i t u d e s 55° and 59°N. Movement outwards from t h i s g r e a t neve was d o m i n a n t l y towards the northwest from n o r t h of l a t i t u d e 59°N, but southeastwards t o the south of l a t i t u d e 54°N a c r o s s the I n t e r i o r P l a t e a u . Ice a l s o s p i l l e d a c r o s s the low passes i n the b o r d e r i n g mountain r a n g e s , moving westwards t o the c o a s t or eastwards to the western p r a i r i e s . The r e t r e a t of the C o r d i l l e r a n i c e was f o l l o w e d , l a t e i n the P l e i s t o c e n e , by a stage d u r i n g which t r u n k g l a c i e r s i n the major v a l l e y s were f e d by a l p i n e g l a c i e r s from w i t h i n the mountains. T h i s r e s u l t e d i n e r o s i o n w i t h i n the v a l l e y s and the rearrangement of e a r l i e r g l a c i a l depos i t s . D a l y ; (1912) r e p o r t e d t h a t a s i m i l a r sequence of events had ta k e n p l a c e i n the area a l o n g the 49th p a r a l l e l . 16 A major g l a c i a t i o n of i c e - s h e e t p r o p o r t i o n s had been f o l -lowed by a p e r i o d of more l o c a l i z e d v a l l e y g l a c i a t i o n . D r y s d a l e (1914) found two t i l l s h e e t s s e p a r a t e d by crossbedded g r a v e l and sand i n the Thompson V a l l e y down-stream from Kamloops Lake. However, the t i l l s and g r a v e l " i n t e r f i n g e r and i n p l a c e s are v e r y d i f f i c u l t t o s e p a r a t e from each o t h e r " ,: so i t i s d o u b t f u l i f two d i s t i n c t g l a c i a -t i o n s o c c u r r e d . Two major advances of the C o r d i l l e r a n i c e sheet s e p a r a t e d by an i n t e r g l a c i a l stage were p o s t u l a t e d by Armstrong and T i p p e r (1948) f o r the Nechako P l a t e a u a r e a . However, they suggest t h a t the i c e a c c u m u l a t i o n zone was l o c a t e d i n the mountains of the Coast Range, i n c o n t r a s t to Dawson's view of a neve c e n t e r e d above the p l a t e a u . In f a c t , they i d e n t i f y s e v e r a l major c e n t e r s of a c c u m u l a t i o n a l o n g t h i s range w i t h the b i g g e s t one i n the Coast Mountains . to the e a s t of Queen C h a r l o t t e Sound. The i n t e r g l a c i a l stage i s not r e p r e s e n t e d i n the mountains, and so may o n l y have o c c u r r e d i n the l o w l a n d a r e a s . In 1954, Armstrong and Brown ( w h i l e summarizing e a r l i e r work on t h i s problem) conclude t h a t over most of B r i t i s h Columbia t h e r e have been at l e a s t two major g l a c i a l s t ages s e p a r a t e d by an i n t e r g l a c i a l e v i d e n c e d by f l u v i a l s ediments. However, each major g l a c i a t i o n may have had more than one l o c a l i c e advance. A l s o t h e r e may have been o t h e r major g l a c i a t i o n s at e a r l i e r dates w i t h no e v i d e n c e r e m a i n i n g at p r e s e n t , and t h e r e was p o s s i b l y a w i d e s p r e a d 17 minor g l a c i a t i o n a t a ve r y l a t e date which would i n c l u d e the Sumas Stade and the v a l l e y g l a c i a t i o n s . The o c c u r r e n c e of a l a t e stage of v a l l e y and a l p i n e g l a c i a t i o n has been v e r i f i e d by many workers i n the areas to the e a s t of the Coast Mountains ( e . g . , R i c e , 1960; H o l l a n d , 19 64). Armstrong and F u l t o n (1965) b r i e f l y des-c r i b e s e v e r a l exposures from v a l l e y s w i t h i n the s o u t h -e a s t e r n p a r t of the I n t e r i o r P l a t e a u where t h e r e i s c l e a r e v i dence f o r two g l a c i a l p e r i o d s and an i n t e r v e n i n g i n t e r -g l a c i a l . However, t h e r e i s no c l e a r e v idence f o r the o c c u r r e n c e of a l a t e f i n a l stage w i t h i n the s o u t h e r n p a r t of the Coast Mountains. Mathews (1951) c o n c l u d e d t h a t low l e v e l c i r q u e s here were never r e - o c c u p i e d by a l p i n e g l a c i e r s s i n c e they were b u r i e d c o m p l e t e l y d u r i n g the C o r d i l l e r a n g l a c i a l maximum, and so t h e i r f o r m a t i o n must p r e - d a t e t h i s t i m e . Thus any l a t e - P l e i s t o c e n e v a l l e y g l a c i e r s i n t h i s r e g i o n must have been f e d d i r e c t l y from the h i g h e r p a r t s of the l o c a l mountains or from the major a c c u m u l a t i o n zones. R e l a t i v e l y few r a d i o - c a r b o n dates have been o b t a i n e d from the I n t e r i o r P l a t e a u area of B r i t i s h Columbia. F i v e dates have been measured on l a c u s t r i n e and f l u v i a l m a t e r i a l i n sequences t h a t r e p r e s e n t major time i n t e r v a l s of i n t e r -g l a c i a l c o n d i t i o n s , see Table 1.3. These dates range from a p p r o x i m a t e l y 21,000 t o 38,000 y e a r s B.P. and would appear to c o r r e s p o n d a p p r o x i m a t e l y w i t h the 0lympia-Quadra I n t e r -g l a c i a l . Assuming t h a t these d a t e s , from t h e i r s c a t t e r e d l o c a t i o n s , r e p r e s e n t c o n t i n u o u s i n t e r g l a c i a l c o n d i t i o n s ( a t TABLE 1.3 RADIO-CARBON DATES FROM INTERIOR BRITISH COLUMBIA G . S . C . # Locat i o n Date M a t e r i a l Sequence C o l l e c t o r 173 Boat Encampment 21,500+300 wood l a c u s t r i n e s i l t H.W. Nasmith 1950 194 275 258 Shuswap Lake 20,230+270 Kamloops M e r r i t t > 32 ,700 > 37 , 200 wood wood s h e l l s l a c u s t r i n e sand and s i l t i n t e r g l a c i a l g r a v e l s i n t e r g l a c i a l v a l l e y f i l l R. J . F u l t o n , 1963 Dyck , F y l e s and B l a k e , 1965 R. J . F u l t o n , 1964 Source: Radiocarbon Supplement, American 1 J o u r n a l of S c i e n c e 19 l e a s t i n the v a l l e y s ) d u r i n g t h i s time i n t e r v a l , i t appears t h a t the Quadra I n t e r g l a c i a l had a l o n g e r d u r a t i o n i n l a n d than at the coast--17 ,000 y e a r s as compared to 11 ,000 y e a r s . T h i s c o u l d p o s s i b l y be e x p l a i n e d by the much c l o s e r p r o x i m i t y of the c o a s t a l l o w l a n d s to the i c e a c c u m u l a t i o n c e n t e r s i n the Coast.Mountains , and hence the e a r l i e r a r r i v a l and l a t e r d i s a p p e a r a n c e of the i c e . • The rainshadow e f f e c t of the Coast Mountains may a l s o have d e l a y e d the e x t e n s i o n of i c e i n t o the I n t e r i o r P l a t e a u a r e a . There would thus be a c l e a r l y d e f i n e d g l a c i a l stage w i t h i n the s o u t h e r n i n t e r i o r of B r i t i s h Columbia, f o l l o w i n g the above mentioned i n t e r g l a c i a l , w h i c h would c o r r e s p o n d t o the F r a s e r G l a c i a t i o n of the Lower F r a s e r V a l l e y and Puget Lowland, and the C l a s s i c a l W i s c o n s i n of e a s t e r n North America. 1.2.3 C o r r e l a t i o n of G l a c i e r F l u c t u a t i o n s Long d i s t a n c e c o r r e l a t i o n of s h o r t term g l a c i e r f l u c t u a t i o n s i s i n a p p r o p r i a t e h e r e , p a r t i c u l a r l y w i t h i n an area as l a r g e and w i t h as v a r i e d c l i m a t e and topography as B r i t i s h Columbia. However, c e r t a i n g e n e r a l i z a t i o n s can be made t h a t may h e l p i n the i n t e r p r e t a t i o n of l o c a l p a t t e r n s : (1) (a) T h e - g r e a t e s t number of g l a c i a l f l u c t u a t i o n s have been r e c o r d e d from areas n e a r e s t to the margin of the i c e s h e e t s , such as the F r a s e r - P u g e t l o w l a n d s . G l a c i e r r e t r e a t s and advances here r e f l e c t s m a l l changes i n g l a c i e r budget which pass u n n o t i c e d i n the areas n e a r e r to the neve where a s l i g h t 20 change i n i c e t h i c k n e s s l e a v e s no t o p o g r a p h i c or s t r a t i g r a p h i c r e c o r d , (b) S t r a t i g r a p h i c e v idence i s b e s t p r e s e r v e d i n areas t h a t are m a r g i n a l to an i c e sheet where d e p o s i t i o n , r a t h e r than e r o s i o n , i s the dominant p r o c e s s . In areas covered by g r e a t e r t h i c k n e s s e s of i c e , e r o -s i o n i s more s e v e r e , and c o n s e q u e n t l y , much evi d e n c e of e a r l i e r g l a c i a l and i n t e r g l a c i a l s t a g e s may be removed by subsequent e r o s i o n . Thus i t i s n e c e s s a r y to d i s t i n g u i s h between cases (a) and (b) above when c o n s i d e r i n g the apparent number of g l a c i a l s t a g e s i n any a r e a . (2) The l a t e stage of v a l l e y and a l p i n e g l a c i a t i o n was p o s s i b l y c o n f i n e d to the h i g h e r mountain m a s s i f s and a d j a c e n t v a l l e y s . Adequate snow s u p p l y on these mountains would promote the development of l o c a l a c c u m u l a t i o n zones f o l l o w i n g the r e t r e a t of the major C o r d i l l e r a n i c e s h e e t . There was thus an u n u s u a l sequence of events d u r i n g the s h i f t from C o r d i l l e r a n t o L o c a l i c e sheet c o n d i t i o n s . (3) C o r r e l a t i o n , i n terms of a b s o l u t e d a t e s , cannot be expected from areas at v a r y i n g d i s t a n c e s from c e n t e r s of i c e a c c u m u l a t i o n and d i s p e r s a l . There w i l l be a c o n s i d e r a b l e time l a g between the b e g i n n i n g of a g l a c i a l stage i n the a r e a a d j a c e n t to an a c c u m u l a t i o n zone and r e g i o n s f u r t h e r away. 21 1.2.4 Ice D i s p e r s a l and Ice Movement During the e a r l y phases of each g l a c i a l stage and the l a t e a l p i n e and v a l l e y phases, the d i r e c t i o n of i c e movement was c o n t r o l l e d by topography. These are the " a l p i n e " and " i n t e n s e a l p i n e " s t a g e s of K e r r (1936),and Phases 1 and 2 of Davis and Mathews (1944). I ce t h i c k n e s s i s l e s s than the l o c a l r e l i e f , t h e r e f o r e i c e f l o w i s c o n f i n e d t o the v a l l e y s and passes a c r o s s low d i v i d e s . As the i c e sheet i n c r e a s e s i n t h i c k n e s s and i n d i v i d u a l v a l l e y g l a c i e r s o v e r t o p t h e i r d i v i d e s t o form an unbroken i c e s h e e t , the f l o w becomes i n c r e a s i n g l y c o n t r o l l e d by the tendency to move outwards from the major c e n t e r s of a c c u m u l a t i o n . At t h i s stage o n l y the b a s a l i c e l a y e r s move i n a c c o r d w i t h the u n d e r l y i n g topography ( i f they move at a l l ) and v e l o c i t i e s are v e r y low. The i c e stream r e a c h e s i t s g r e a t e s t v e l o c i t i e s where the i c e i s t h i n n e s t and moving a c r o s s convex s l o p e s , as a c r o s s d i v i d e s or mountain r i d g e s . The r e l a t i v e movement i n each case has some b e a r i n g on the amount of e r o s i o n t h a t w i l l , be a c c o m p l i s h e d (Mathews, 1944). E v i d e n c e from s t r i a e and i c e moulded forms on the upland s u r f a c e of the I n t e r i o r P l a t e a u , i n d i c a t e s t h a t i n s o u t h - c e n t r a l B r i t i s h Columbia i c e movement was c o n t r o l l e d by d i s p e r s a l from the. Coast Mountains and from an " i c e - s h e d " t r e n d i n g from e a s t - n o r t h - e a s t to west-south-west a p p r o x i -mately through C l i n t o n ( G e o l . Assn. of Can., 1958). These e f f e c t s combined to produce an o v e r a l l movement towards the s o u t h - s o u t h - e a s t over the s o u t h e r n s e c t i o n of the p r o v i n c e . 22 However, l o c a l v a r i a t i o n s i n d i r e c t i o n r e s u l t e d when the i c e stream was c o n t r o l l e d by o u t s t a n d i n g t o p o g r a p h i c f e a -t u r e s . For example, the n o r t h - s o u t h t r e n d i n g F r a s e r V a l l e y e f f e c t i v e l y c h a n n e l l e d a p o r t i o n of the i c e sheet i n t o a s o u t h e r l y d i r e c t i o n , but the Thompson V a l l e y , t r e n d i n g e a s t -west, t r a n s v e r s e to the g e n e r a l f l o w d i r e c t i o n , had l i t t l e or no e f f e c t . U n u s u a l l y h i g h s e c t i o n s of the I n t e r i o r P l a t e a u a l s o had a tendency to d i v e r t the i c e — t h e n o r t h e r n end of the Okanagan Range near P r i n c e t o n s p l i t the i c e sheet i n t o two l o b e s w i t h r e g a r d t o movement, one l o b e moved down the lower- Similkameen V a l l e y , w h i l s t the o t h e r f l o w e d s o u t h -wards between the upper Similkameen and Tulameen ( R i c e , 1960). 1.2.5 Ice T h i c k n e s s During the stage of maximum g l a c i a t i o n the i c e com-p l e t e l y b u r i e d the I n t e r i o r P l a t e a u w h i l e the h i g h e r p o r t i o n s of a d j a c e n t mountain ranges remained as nunatak a r e a s . The i c e s u r f a c e s l o p e d from 8,000' j u s t n o r t h of the l a t i -tude of Kamloops t o 7,000' at the I n t e r n a t i o n a l Boundary and a l o n g the w e s t e r n f l a n k of the Cascades between L y t t o n and Hope ( G e o l . Assn. of Can., 1958). T h e r e f o r e t h e r e was a maximum t h i c k n e s s of about 2,000' of i c e over the p l a t e a u s u r f a c e , t h i c k e n i n g to 6,000' or 7,000' over the major v a l l e y s such as the F r a s e r , Thompson, Okanagan and Similkameen. The i c e s u r f a c e was h i g h e r towards the mountains; g l a c i a l e r r a t i c s have been found at 8,600' i n the Okanagan Range ( R i c e , 1960) and at 8,300' i n the Coast 23 Mountains near Texas Creek ( D u f f e l l and McTaggart, 1952). 1.2.6 P o s t - G l a c i a l H i s t o r y D u r i n g i c e r e t r e a t i r r e g u l a r m e l t i n g commonly l e f t i c e b l o c k i n g major v a l l e y s w h i l s t a d j a c e n t areas were i c e f r e e . P r o - g l a c i a l l a k e s r e s u l t e d i n t o which s i l t , sand and g r a v e l were d e p o s i t e d . A g e n e r a l p e r i o d of a g g r a d a t i o n a l s o commenced at t h i s t i m e , and c o n t i n u e d f o r some time a f t e r the i c e had f i n a l l y d i s a p p e a r e d . The f l o o r s of most major v a l l e y s were b u r i e d by s e v e r a l hundred f e e t of c l a s t i c m a t e r i a l , which i n c l u d e d a l l u v i a l fans and cones, t a l u s s l o p e s and a l l v a r i e t i e s of c o l l u v i u m . In many v a l l e y s a f i n a l phase of d o w n c u t t i n g by the r i v e r s d i s s e c t e d the e a r l i e r c o n s t r u c t i o n a l s u r f a c e s . The p h y s i c a l c h a r a c t e r of a p a r t i c u l a r r e g i o n at the p r e s e n t time appears to be l a r g e l y dependent upon i t s p e c u l i a r p a t t e r n of d e g l a c i a t i o n and the subsequent phases of a g g r a d a t i o n and d e g r a d a t i o n . For t h i s r e a s o n , d e t a i l e d d i s c u s s i o n s of p o s t - g l a c i a l h i s t o r y are c o n f i n e d t o the s e p a r a t e r e g i o n a l c h a p t e r s t h a t f o l l o w . CHAPTER I I THE PLEISTOCENE GLACIATION OF STUDY AREAS The f i v e s tudy areas d i f f e r i n d e t a i l w i t h r e g a r d t o the r e s u l t s of the P l e i s t o c e n e g l a c i a t i o n . The c o n t r o l s t h a t gave r i s e t o t h i s d i f f e r e n t i a t i o n i n c l u d e the o r i e n t -a t i o n of the major v a l l e y s , t h e i r d i s t a n c e from the major c e n t e r s of i c e a c c u m u l a t i o n and the l o c a l topography. The p a t t e r n of d e g l a c i a t i o n t h a t r e s u l t e d from these f a c t o r s a l s o enhanced the i n d i v i d u a l c h a r a c t e r of each r e g i o n . 2.1 G l a c i a l E f f e c t s w i t h i n the Study Areas 2.1.1 The F r a s e r V a l l e y Any attempt to e s t i m a t e the amount of g l a c i a l e r o s i o n t h a t took p l a c e here i s n e c e s s a r i l y h i g h l y t e n t a t i v e s i n c e t h e r e i s no d i r e c t e v i d e n c e of any g l a c i a l deepening. Indeed, the o n l y v i s i b l e e f f e c t s of the g l a c i a t i o n w i t h i n the v a l l e y are the smooth, i c e - a b r a d e d s u r f a c e s found on the g r a n o d i o r i t e of the western v a l l e y s i d e . G l a c i a l deepening of the v a l l e y by c o n s i d e r a b l y l e s s than 2,000' may be i n f e r r e d from the e s t i m a t e made f o r the t o t a l amount of e r o s i o n t h a t f o l l o w e d the P l i o c e n e u p l i f t ( c f . , 1.1.2). The p r o x i m i t y of t h i s a r ea to the Coast Mountains i s s i g n i f i c a n t f o r s e v e r a l r e a s o n s . F i r s t l y , s i n c e the i c e movement d u r i n g the C o r d i l l e r a n phase was outwards from the a c c u m u l a t i o n zones (which i n c l u d e d t h i s s e c t i o n of the Coast 25 Range), the tendency was f o r i c e to move a c r o s s t h i s p a r t of the F r a s e r V a l l e y r a t h e r than a l o n g i t . S t r i a e from summits and r i d g e s a d j a c e n t to the v a l l e y i n d i c a t e a g e n e r a l movement towards S 30°E ( D u f f e l l and McTaggart , 1952). However, w i t h the i c e s u r f a c e s t a n d i n g above 8,000' ( G l a c i a l Map of Canada, 1958), t h e r e must have been a t h i c k -ness of more than 7,000' of i c e above the v a l l e y f l o o r , so the b a s a l l a y e r s may have had v e r y low v e l o c i t i e s . Movement here would p r o b a b l y have been i n the down v a l l e y d i r e c t i o n , w h i l e the f a s t e r moving s u r f a c e i c e was f o r c e d towards the s o u t h - e a s t . E r o s i o n of the v a l l e y f l o o r was p r o b a b l y s l i g h t d u r i n g the C o r d i l l e r a n phase, but the e f f e c t of the t r a n s v e r s e i c e movement at g r e a t e r e l e v a t i o n s may have been severe e r o s i o n on the upper p a r t of the e a s t e r n v a l l e y s i d e s where i c e v e l o c i t i e s would be h i g h e r . A l s o i t i s p o s s i b l e t h a t the r e l a t i v e l y broad s e c t i o n of the F r a s e r V a l l e y at L i l l o o e t r e s u l t e d from i n c r e a s e d e r o s i o n by r a p i d l y moving i c e i n a r e s t r i c t e d a rea t h a t was r e c e i v i n g two l a r g e i c e streams from the B r i d g e and Seton V a l l e y s . S e c o n d l y , s i n c e t h i s a rea i s so c l o s e to the accumu-l a t i o n zone, i t i s l i k e l y to have e x p e r i e n c e d fewer g l a c i a l f l u c t u a t i o n s than r e g i o n s f a r t h e r to the e a s t , and i t i s a l s o l i k e l y t h a t i c e o c c u p i e d t h i s a rea c o n t i n u o u s l y (as a v a l l e y g l a c i e r d u r i n g the l a t e r phases) u n t i l a r e l a t i v e l y l a t e d a t e . The most i m p o r t a n t e f f e c t of t h i s l a t e d e g l a c i a t i o n was the b l o c k i n g of the s o u t h e r l y f l o w i n g 26 Thompson d r a i n a g e by an i c e l o b e e x t e n d i n g up the lower p a r t of the Thompson V a l l e y from the F r a s e r V a l l e y (Mathews, 1944). There i s evidence f o r c o n s i d e r a b l e g l a c i a l e r o s i o n i n the h i g h e r p a r t s of the Coast Mountains i n the headwater v a l l e y s of the F r a s e r t r i b u t a r i e s . Ice covered these moun-t a i n s t o a h e i g h t of at l e a s t 8,300' ( D u f f e l l and McTaggart, 1952), y e t some c i r q u e s and o t h e r a l p i n e g l a c i a l f e a t u r e s are c l e a r l y d e f i n e d at e l e v a t i o n s below t h i s . T h e r e f o r e they may have been r e o c c u p i e d a f t e r the waning of the major i c e s h e e t , and the a l p i n e phase c o n t i n u e d f o r a c o n s i d e r a b l e l e n g t h of t i m e - - i n f a c t to the p r e s e n t day i n the case of the s m a l l c i r q u e g l a c i e r s on the f l a n k s of S t e i n and Siwhe mountains. Ice from these a l p i n e g l a c i e r s p o s s i b l y c o l l e c t e d i n the " g u t t e r - 1 i k e " F r a s e r V a l l e y t o form the v a l l e y - g l a c i e r stage h e r e , a l t h o u g h a l t e r n a t e l y , a F r a s e r V a l l e y g l a c i e r c o u l d a l s o be the s i m p l e r e s u l t of the a b l a t i o n of the i c e she e t . 2.1.1 The Thompson V a l l e y T h i s a r e a was c o m p l e t e l y b u r i e d by i c e d u r i n g the g l a c i a l maxima. D r i f t - c o v e r e d and s t r i a t e d s u r f a c e s have been found at the h i g h e s t e l e v a t i o n s . The i c e s u r f a c e stood at 8,000' ( G l a c i a l Map of Canada, 1958) and so the maximum i c e t h i c k n e s s would be 7,000' over the Thompson V a l l e y , t h i n n i n g to a minimum of about 2,000' over the h i g h e s t a d j a c e n t summits. The o v e r a l l d i r e c t i o n of i c e 27 movement f o l l o w e d the r e g i o n a l p a t t e r n , but the b a s a l l a y e r s ( o r the t h i n n i n g i c e ) d i d move w i t h the g e n e r a l t r e n d of the v a l l e y . E v i d e n c e f o r t h i s i s p r o v i d e d by an area of i c e scoured and s t r e a m l i n e d topography l o c a t e d on the g e n t l e western s l o p e of the Thompson V a l l e y between Spatsum and A s h c r o f t . The area c o n s i s t s of low d r u m l i n o i d h i l l s composed of bedrock. T h e i r s t e e p e r ends f a c e n o r t h -wards; t h e i r l o n g axes are s u b - p a r a l l e l and t r e n d n o r t h -south p a r a l l e l t o the main v a l l e y . 2.1.3 The Bonaparte V a l l e y The a l i g n m e n t of t h i s v a l l e y i s a p p r o x i m a t e l y p a r a l l e l t o the r e g i o n a l d i r e c t i o n of i c e movement as i n d i c a t e d by g l a c i a l s t r i a e ( D u f f e l l and McTaggart , 19 52 ; - G l a c i a l Map of Canada, 1958). Thus the movement of the e n t i r e body of i c e , both b a s a l and s u r f i c i a l l a y e r s , would have been i n the down v a l l e y d i r e c t i o n . T h i s s i t u a t i o n would be most conducive to g l a c i a l a b r a s i o n . 2.1.4 The Similkameen V a l l e y D u r i n g the g l a c i a l maxima the i c e s u r f a c e s t o o d at more than 8,500' above the Similkameen V a l l e y and s t r i a e i n d i c a t e t h a t the movement f o l l o w e d the r e g i o n a l t r e n d , from no r t h w e s t to s o u t h e a s t , a c r o s s the h i g h e s t l a n d . During a l a t e r and l e s s e r g l a c i a l stage t h e r e i s e v i d e n c e t h a t the i c e f l o w f o l l o w e d more c l o s e l y the n o r t h - s o u t h t r e n d of the major v a l l e y , w i t h an i c e stream between 3 and 4- m i l e s i n w i d t h f l o w i n g a l o n g the e a s t e r n s i d e of the 28 Okanagan Range ( R i c e , 1947). W i t h i n the v a l l e y , some l o c a t i o n s were so s h e l t e r e d from i c e a b r a s i o n by r i d g e s t r a n s v e r s e to the d i r e c t i o n of i c e movement, t h a t weathered bedrock remains>in_ s i t u t o the p r e s e n t t i m e . Such an area i s the western v a l l e y s i d e between Susap and Snehumption c r e e k s ; the w h i t e , s i l t y appearance of the ground i n d i c a t e s the presence of the weathered bedrock l a y e r ( B o s t o c k , 1940). A l t h o u g h the a c t u a l t h i c k n e s s of the weathered zone i s not known, i t may e i t h e r be a s s i g n e d to p r e - G l a c i a l t i m e , or to a l o n g l a s t i n g i n t e r g l a c i a l s i n c e i t s o c c u r r e n c e i s so l o c a l i z e d ( i m p l y i n g t h a t much weathered m a t e r i a l was removed d u r i n g g l a c i a t i o n ) , and a l s o , the weathered m a t e r i a l appears to be the p r o d u c t of c h e m i c a l , r a t h e r than p h y s i c a l w e a t h e r i n g p r o c e s s e s and i t i s the l a t t e r t h a t have.been most a c t i v e d u r i n g Recent t i m e . The o c c u r r e n c e of a l a t e phase of a l p i n e and/or v a l l e y g l a c i a t i o n here i s i n d i c a t e d by the e x i s t e n c e of " f r e s h " c i r q u e s and r e l a t e d f e a t u r e s at h i g h e l e v a t i o n s i n the Okanagan Range. These must p o s t - d a t e the i c e sheet phase and have been r e o c c u p i e d d u r i n g i c e a c c u m u l a t i o n and d i s p e r s a l w i t h i n the Okanagan Range. P o s s i b l y a v a l l e y g l a c i e r o c c u p i e d the main Similkameen V a l l e y at t h i s time and was f e d by the w e s t e r n t r i b u t a r y v a l l e y s . The Thompson P l a t e a u to the e a s t shows no landforms of a l p i n e g l a c i a t i o n and was presumably too low f o r i c e a c c u m u l a t i o n . 29 2.1.5 The Kamloops Area The east-west t r e n d of the Thompson and South Thompson v a l l e y s was t r a n s v e r s e to the r e g i o n a l i c e movement from nort h w e s t towards the s o u t h e a s t . S t r i a e f o l l o w the r e g i o n a l t r e n d and t h e r e i s no i n d i c a t i o n t h a t t h i s was m o d i f i e d by the v a l l e y topography ( G e o l . Assn. of Can., 1958). I t may be t e n t a t i v e l y suggested t h a t the movement of the C o r d i l l e r a n i c e sheet a c r o s s the t r a n s v e r s e v a l l e y may have produced, an a s y m m e t r i c a l c r o s s p r o f i l e due to i n c r e a s e d e r o s i o n on the s o u t h e r n , or d o w n f l o w ; " • v a l l e y s i d e where the i c e would be t h i n n i n g and moving a g a i n s t the s l o p e of the u n d e r l y i n g s u r f a c e . 2.2 D e g l a c i a t i o n of the Fraser-Thompson Drainage System S i n c e f o u r of the f i v e study areas l i e w i t h i n the Fraser-Thompson d r a i n a g e system, a g e n e r a l d i s c u s s i o n of the d e g l a c i a t i o n of t h i s r e g i o n i s a p p r o p r i a t e here. The area of i n t e r e s t i n c l u d e s t h a t p o r t i o n of the South Thompson V a l l e y downstream from Shuswap Lake, the most s o u t h e r l y s e c t i o n of the North Thompson V a l l e y , the Thompson V a l l e y from Kamloops to L y t t o n and the t r i b u t a r y Bonaparte V a l l e y , and the study area s e c t i o n of the F r a s e r V a l l e y . As a r e s u l t of the t o p o g r a p h i c form of the I n t e r i o r P l a t e a u and the consequent g r e a t v a r i a t i o n i n t h i c k n e s s of the c o n t i n e n t a l i c e sheet between v a l l e y and p l a t e a u a r e a s , the p a t t e r n of i c e r e t r e a t was marked by two r a t h e r u n u s u a l 30 c h a r a c t e r i s t i c s . F i r s t l y , the i c e f r o n t was e x t r e m e l y i r r e g u l a r (Mathews, 19 4 4) , i n c o n t r a s t to the more u s u a l b r o a d l y l o b a t e p a t t e r n s of the margins of c o n t i n e n t a l i c e s h e e t s . S e c o n d l y , w i t h the d e c l i n e i n i c e f l o w from the a c c u m u l a t i o n zones, the i c e mass " s t a r v e d on the I n t e r i o r P l a t e a u and d i s i n t e g r a t e d l a r g e l y by downwasting" ( F u l t o n , 1967, p. 2 ) ; the upland areas of p l a t e a u thus became i c e -f r e e b e f o r e the major v a l l e y s . A c r o s s the i n t e r i o r o f s o u t h e r n B r i t i s h Columbia the g e n e r a l form of the r e t r e a t i n g i c e f r o n t was t h a t of an arc concave to the s o u t h . The i c e f r o n t r e t r e a t e d towards the n o r t h , n o r t h w e s t and n o r t h e a s t where the major a c c u m u l a t i o n zones and areas of i c e d i s p e r s a l were l o c a t e d (Mathews, 1944). As a consequence of these v a r i o u s a s p e c t s of d e g l a c i a t i o n , numerous g l a c i a l l a k e s were formed a g a i n s t the r e t r e a t i n g edge of the i c e sheet or a g a i n s t s t a g n a t i n g i c e b l o c k s o c c u p y i n g major v a l l e y s . Thus the p a t t e r n of i c e r e t r e a t e x e r t e d a s t r o n g c o n t r o l upon the l a t e r 'sequence of d e p o s i t i o n and development of landforms w i t h i n the v a l l e y s . 2.2.1 G l a c i a l Lake Thompson The ice-dammed l a k e named Lake Thompson by Mathews (19 4 4 ) , o c c u p i e d a l a r g e p a r t of the Thompson V a l l e y and was the s i t e of major s e d i m e n t a t i o n . A l t h o u g h the sequence of development f o r t h i s l a k e t h a t i s o u t l i n e d below most l i k e l y took p l a c e d u r i n g the f i n a l d e g l a c i a t i o n of the 31 Thompson b a s i n , t h e r e i s e v i d e n c e t h a t at l e a s t one i n t e r -g l a c i a l l a k e e x i s t e d w i t h i n the same a r e a . 1 P o s s i b l y Lake Thompson i s the most r e c e n t of s e v e r a l g l a c i a l l a k e s w i t h s i m i l a r h i s t o r i e s . The f o l l o w i n g account i s based c h i e f l y upon the work of W. H. Mathews (1944, 1960) and R. J . F u l t o n (1963, 1965). (1) Lake Thompson - I n i t i a l Stage D u r i n g the g e n e r a l n o r t h e r l y r e t r e a t and t h i n n i n g of the i c e sheet i c e remained i n the v a l l e y s of the Thompson, No r t h Thompson and South Thompson R i v e r s w h i l e the a d j a c e n t upland areas were r e l a t i v e l y i c e f r e e (Mathews, 1944; F u l t o n , 1965). The i c e tongue i n -the South Thompson V a l l e y s l o p e d eastwards and c a r r i e d m a r g i n a l d r a i n a g e to c o l s l e a d i n g t o the Okanagan V a l l e y . A s e r i e s of kame t e r r a c e s a l o n g the s o u t h e r n w a l l of the South Thompson V a l l e y were c o n s t r u c t e d at t h i s time ( F u l t o n , 1963). S u b s e q u e n t l y , the South Thompson i c e tongue s p l i t i n the v i c i n i t y of Monte Creek and Lake Thompson o r i g i n a t e d between the e a s t e r l y and w e s t e r l y r e t r e a t i n g i c e l o b e s , F i g . 2.1 (Mathews, 1944; F u l t o n , 1963). (2) Lake Thompson - 1,650' Stage Lake Thompson expanded as the c o n f i n i n g i c e l o b e s r e t r e a t e d ; the e a s t e r l y l o b e i n t o the b a s i n of the p r e s e n t Shuswap. Lake, and the w e s t e r l y l o b e s e p a r a t i n g near Kamloops i n t o a n o r t h e r l y r e t r e a t i n g i c e mass o c c u p y i n g the North "'"Armstrong and F u l t o n ( 1965 , p. 99) d e s c r i b e i n t e r -g l a c i a l l a c u s t r i n e beds near Kamloops. GLACIAL LAKE THOMPSON - LOCATION MAP 33 Thompson V a l l e y , and a more r a p i d l y r e t r e a t i n g w e s t e r l y l o b e o c c u p y i n g the b a s i n of the p r e s e n t Kamloops Lake. A l t h o u g h numerous l a k e m a r g i n a l f e a t u r e s have been i d e n t i f i e d i n the Kamloops a r e a at e l e v a t i o n s r a n g i n g from 1,400' to 1 ,800' , two s t r a n d l i n e s are p a r t i c u l a r l y prominent at 1,400' and 1,650' (Mathews, 1944; F u l t o n , 1963, 1965). A l a k e s tage s t a n d i n g at 1,650' o c c u p i e d the Thompson-South Thompson V a l l e y between the we s t e r n end of Kamloops Lake and L i t t l e Shu'swap Lake. T h i s l a k e d r a i n e d eastwards i n t o the Okanagan system v i a a c o l near S q u i l a x , F i g . 2.1. P o s t - g l a c i a l i s o s t a t i c adjustment has t i l t e d the s t r a n d l i n e of t h i s l a k e 6.3' per m i l e on a N 60° W i s o b a s e . S e v e r a l prominent d e p o s i t i o n a l forms are a s s o c i a t e d w i t h t h i s stage of Lake Thompson. The South Thompson S i l t , i n v e s t i g a t e d by F u l t o n (1963, 1965) i s a s t r a t i g r a p h i c u n i t p o s s i b l y up to 500' i n t h i c k n e s s and o c c u p y i n g the South Thompson V a l l e y from Kamloops to a p o i n t 35 m i l e s to the e a s t . D e p o s i t i o n of a l a r g e g r a v e l d e l t a at the mouth of the Deadman R i v e r at the western end of Kamloops Lake commenced d u r i n g t h i s stage of Lake Thompson and c o n t i n u e d d u r i n g l a t e r and lower s t a g e s (Armstrong and F u l t o n , 1965). (3) Lake Thompson - 1 ,400' Stage The l e v e l of Lake Thompson f e l l as i c e m e l t i n g uncovered lower o u t l e t s on the Thompson-Okanagan d i v i d e . S i n c e the s u r f a c e of the South Thompson White S i l t i s above 1 ,400' , d r a i n a g e of Lake Thompson was now by a slow-f l o w i n g r i v e r eastwards through the South Thompson V a l l e y 34 t h a t commenced d i s s e c t i o n of the w h i t e s i l t body as the l a k e l e v e l f e l l , F i g . 2.1. P o s t - g l a c i a l i s o s t a t i c a d j u s t -ment has t i l t e d the 1,400' s t r a n d l i n e 3.6' per m i l e on the N 60° W i s o b a s e . Large b o d i e s of sediment were d e p o s i t e d i n t o t h i s stage of Lake Thompson. They i n c l u d e the s m a l l g r a v e l d e l t a at the mouth of P a u l Creek, llg m i l e s n o r t h of Kamloops, a l a r g e and complex d e l t a i c sequence at the mouth of the Bonaparte R i v e r near A s h c r o f t , and a second s e r i e s of w h i t e s i l t s i n the Thompson V a l l e y between A s h c r o f t and Spences B r i d g e , F i g . 2.1. A more d e t a i l e d d i s c u s s i o n of the s e d i -m e n t a t i o n i n t h i s r e g i o n i s i n c l u d e d i n Chapter IV. Dur i n g the e x i s t e n c e of the 1,400' l a k e s t a g e , the i c e b a r r i e r r e t r e a t e d down the Thompson V a l l e y from s l i g h t l y n o r t h of A s h c r o f t (Mathews, 1944; Armstrong and F u l t o n , 1965) t o s l i g h t l y south of Spences Bridge.- As the i c e lobe c l e a r e d the Nicola-Thompson c o n f l u e n c e at Spences B r i d g e a g l a c i a l l a k e i n the N i c o l a V a l l e y was l i n k e d t o Lake Thompson. With the f i n a l m e l t i n g of the i c e i n the Thompson V a l l e y s o u t h e r l y d r a i n a g e of the Thompson system was e s t -a b l i s h e d and the l a k e phase t e r m i n a t e d . 2.2.2 D e g l a c i a t i o n of the Fraser-Thompson D i v i d e Ice s u p p l i e d from the Coast Mountains m a i n t a i n e d the g l a c i a l stage i n the F r a s e r V a l l e y u n t i l a l a t e r date than i n the a d j a c e n t Thompson V a l l e y . I t may be supposed t h a t 35 d u r i n g the 1,400' stage of Lake Thompson, the F r a s e r V a l l e y was s t i l l o c c u p i e d by an a c t i v e g l a c i e r w i t h a tongue e x t e n d i n g up the lower Thompson V a l l e y to form the i c e dam t h e r e . P o s s i b l y , the e x t r e m i t y of t h i s l o b e c o n s i s t e d of s t a g n a t i n g i c e . The upper p o r t i o n of the Thompson V a l l e y was i c e f r e e and d r a i n i n g eastwards to the Okanagan. C o n s e q u e n t l y , i t may be supposed t h a t e a r l y m e l t w a t e r d r a i n a g e from the F r a s e r i c e sought out r o u t e s of escape v i a the open Thompson V a l l e y . S i n c e the F r a s e r i c e was moving outwards from the Coast Mountains and/or b e i n g r a p i d l y melted on the I n t e r i o r P l a t e a u , i t may be assumed t h a t the i c e s u r f a c e s l o p e d downwards towards the e a s t . Thus i n the e a r l y s t a g e s of d e g l a c i a t i o n of the F r a s e r b a s i n , m eltwater from the Coast Mount'-: ain.s' -and ' F r a s e r g l a c i e r system would have been ponded between the e a s t e r n edge of the i c e sheet and the h i g h ground of the Fraser-Thompson d i v i d e . The u p l a n d area between the Thompson and F r a s e r V a l l e y s c o n t a i n s a maze of c o l s t h a t p o s s i b l y f u n c t i o n e d as s p i l l w a y s f o r the ponded m e l t w a t e r or f o r m e l t w a t e r i s s u i n g d i r e c t l y from the i c e f r o n t . These c o l s are a l s o a s s o c i a t e d w i t h s e v e r a l cases of r i v e r c a p t u r e and d r a i n a g e . d i v e r s i o n s . Some of the more a c c e s s i b l e c o l s were examined by the w r i t e r ; they t y p i c a l l y have f l a t f l o o r s , steep s i d e s l o p e s and commonly c o n t a i n l a k e s . As the i c e s u r f a c e was lowered by a b l a t i o n and ground was uncovered at lower and lower e l e v a t i o n s , m e l t w a t e r was 36 a b l e to escape a c r o s s c o n t i n u a l l y lower p o i n t s on the watershed between the F r a s e r and Thompson b a s i n s . Thus, i n g e n e r a l the h i g h e s t c o l s were the f i r s t to be used, and the c o l s at lower e l e v a t i o n s mark s u c c e s s i v e l y l a t e r s t a g e s i n the sequence of e v e n t s . A l t h o u g h i t i s not y e t p o s s i b l e to o u t l i n e the p r e c i s e o r d e r i n which the c o l s were u t i -l i z e d , they can be c o n v e n i e n t l y s u b d i v i d e d i n t o two groups t h a t r e p r e s e n t an e a r l i e r and a l a t e r s tage i n the d e g l a c i a -t i o n of each v a l l e y . Stage One The e a s t e r n edge of the F r a s e r i c e s t o o d at e l e v a t i o n s g e n e r a l l y above 4,500', b l o c k i n g m a r g i n a l l a k e s i n v a l l e y s t r i b u t a r y to the F r a s e r at s i m i l a r e l e v a t i o n s . S p i l l w a y s i n the c o l s c a r r i e d d r a i n a g e i n t o the Thompson b a s i n , F i g . 2.2. The l a r g e s t and l o w e s t c o l i n t h i s s e r i e s i s the P a v i l i o n Creek-Hat Creek v a l l e y t h a t c o n t a i n s P a v i l i o n and Crown Lakes. Maximum e l e v a t i o n of t h i s routeway i s 2,750'. T h i s v a l l e y appears s u f f i c i e n t l y l a r g e to c o n t a i n the e n t i r e F r a s e r R i v e r - - a n d such may have been the case i f the F r a s e r V a l l e y to the south of P a v i l i o n was b l o c k e d by i c e , but i c e f r e e f u r t h e r upstream. However, i n p l a c e s the g e n e r a l l y f l a t f l o o r of t h i s s p i l l w a y i s i n t e r r u p t e d by i r r e g u l a r r i d g e s of t i l l which do not appear to have been m o d i f i e d by any f l u v i a l a c t i o n . T h i s would i n d i c a t e t h a t d u r i n g the f i n a l d e g l a c i a t i o n t h i s c o l was b l o c k e d by i c e and d i d not f u n c t i o n as a major s p i l l w a y . FIGURE 2.2 POSSIBLE GLACIAL SPILLWAY COLS UPON THE FRASER - THOMPSON DIVIDE \ 38 Stage Two The l e v e l of the F r a s e r i c e f e l l below t h a t of the lowest c o l s on the. Fraser-Thompson watershed. Drainage i n both v a l l e y s was e i t h e r a l o n g the v a l l e y w a l l s m a r g i n a l to the i c e or i n a d i r e c t i o n r o u g h l y p a r a l l e l to the main v a l l e y s but v i a s p i l l w a y c o l s l i n k i n g t r i b u t a r y v a l l e y s . These c o l s are t e n t a t i v e l y l i n k e d i n F i g . 2.2 t o show a p o s s i b l e d r a i n a g e p a t t e r n f o r t h i s t i m e . Once the i c e i n the v a l l e y s f e l l below the l e v e l of the l o w e s t c o l s , a l l d r a i n a g e was e i t h e r c o n f i n e d to l a t e r a l c h annels between i c e and v a l l e y s i d e s , or f l o w e d over the s u r f a c e of the i c e i t s e l f . Below the e l e v a t i o n s of the c o l s mentioned above, t h e r e i s o n l y one o t h e r c o l s which c o u l d have c a r r i e d m e l t -water away from the F r a s e r d r a i n a g e system. T h i s i s l o c a t e d at B i r k e n and connects t h e , S e t o n V a l l e y w i t h the L i l l o o e t R i v e r v a l l e y at an e l e v a t i o n of a p p r o x i m a t e l y 1 , 600' . Q u i t e p o s s i b l y t h i s l e v e l i s w e l l below some of the major p r o g l a c i a l l a k e s t h a t were impounded i n the F r a s e r V a l l e y , but due t o i t s p o s i t i o n i n the h e a r t of the Coast M o u n t a i n s , i t seems u n l i k e l y t h a t t h i s c o l would be i c e f r e e b e f o r e the F r a s e r V a l l e y i t s . e l f . F i n a l m e l t i n g of the i c e f o l l o w e d v a r y i n g p a t t e r n s i n each v a l l e y . C o n s e q u e n t l y , they are d i s c u s s e d s e p a r a t e l y from t h i s stage onwards (see Chapters I I I to V I ) . CHAPTER I I I THE FRASER VALLEY - DEVELOPMENT OF CONTEMPORARY LANDFORMS 3 . 1 I n t r o d u c t i o n The 60 m i l e s e c t i o n of the F r a s e r V a l l e y between P a v i l i o n and L y t t o n i n c l u d e s p a r t s of two major p h y s i o g r a p h i c r e g i o n s . From P a v i l i o n t o the mouth of the B r i d g e R i v e r the F r a s e r f l o w s i n a deeply e n t r e n c h e d v a l l e y cut through the ranges t h a t c o n s t i t u t e the r i m of the I n t e r i o r P l a t e a u ; between the mouth of the B r i d g e R i v e r and L y t t o n , the F r a s e r f o l l o w s the boundary between the Coast Mountains to the west and the C l e a r Range ( o f the I n t e r i o r P l a t e a u ) t o the e a s t , F i g . 3.1. T h i s boundary l i n e i s the F r a s e r R i v e r f a u l t zone a l o n g which the e l e v a t i o n of the Coast ranges r e l a t i v e t o the I n t e r i o r P l a t e a u took p l a c e ( c f . , 1.1.1). Along t h i s boundary zone, t h e r e are c o n s i d e r a b l e t o p o g r a p h i c and g e o l o g i c c o n t r a s t s between the e a s t e r n and we s t e r n s i d e s of the F r a s e r V a l l e y . The Coast Mountains here c o n s i s t c h i e f l y of g r a n o d i o r i t e o c c u r r i n g as Cretaceous i n t r u s i o n s , b o r d e r e d by remnants of Mesozoic sediments--cons i s t i n g m a i n l y of a r g i l l i t e s , q u a r t z i t e s and c o n g l o m e r a t e s , F i g . 3.2 ( D u f f e l l and McTaggart, 1952). Summit l e v e l s w i t h i n these mountains are u s u a l l y i above 8 , 000'. A l t h o u g h a g e n e r a l accordance of summit l e v e l s over l a r g e areas i s a p p a r e n t , i n d i v i d u a l peaks are sharp and show a l l c h a r a c t e r i s t i c s of a l p i n e FIGURE 3.2 FRASER VALLEY - BEDROCK GEOLOGY LEGEND TERTIARY Miocene or Earlier KAMLOOPS GROUP 13 Basalt, andesite and rhyolite; associated tuffs and breccias Miocene and Eocene 12 COLDWATER BEDS ( ): sandstone, shale and conglomerate', other rock types CRETACEOUS SPENCES BRIDGE GROUP II Andesite, dacite, basalt and rhyolite, tuff, breccia and agglomerate', JACKASS MOUNTAIN GROUP conglomerate, sandstone and arkose Greywacke, argil lite, arkose and conglomerate LILLOOET GROUP 9 Argillite, volcanic conglomerate and tuffaceous sandstone BREW GROUP 8 Argillite, quartzite and conglomerate TRIASSIC OR EARLIER 7 Phyllite, quartzite, limestone, greenstone, schist and others Schist and gneiss PERMIAN AND ( ? ) EARLIER CACHE CREEK GROUP MARBLE CANYON FORMATION: limestone 4 Greenstone, chert, argillite, minor limestone and quartzite -, chlorite and quartz-mica schist INTRUSIVE ROCKS CRETACEOUS OR LATER Lower Cretaceous or Later - 3 Quartz diorite, albite syenite CRETACEOUS Lower Cretaceous MOUNT LYTTON BATHOLITH: granodiorite, quartz diorite and diorite JURASSIC OR CRETACEOUS > Lower Cretaceous or Earlier 11111 GUICHON CREEK BATHOLITH: granite, granodiorite, quartz diorite and diorite F R A S E R V A L L E Y BEDROCK GEOLOGY After Duffell and McTaggart, 1952 FIGURE 3.2 heavily drift covered areas > faults 42 g l a c i a t i o n w i t h s m a l l g l a c i e r s o c c u p y i n g the h i g h e r l e v e l c i r q u e s at the p r e s e n t t i m e . The F r a s e r R i v e r r e c e i v e s i t s major t r i b u t a r i e s from the west, n o t a b l y the S t e i n R i v e r , Texas Creek, Seton R i v e r and the B r i d g e R i v e r , F i g . 3.1. To the e a s t of the F r a s e r the C l e a r Range ( i n c l u d i n g F o u n t a i n Ridge) a t t a i n s e l e v a t i o n s of 7,000' w i t h summits t h a t tend t o be rounded u n l e s s f o r m i n g the upper p a r t s of the w a l l s of the F r a s e r V a l l e y . G e o l o g i c a l l y , these mountains are more v a r i e d than the a d j a c e n t Coast Mountains and c o n s i s t c h i e f l y of v o l c a n i c and s edimentary r o c k s . The Spences B r i d g e group of v o l c a n i c r o c k s t o g e t h e r w i t h the J a c k a s s Mountain group of Lower Cretaceous' sediments are the major components of the C l e a r Range and extend n o r t h -wards a c r o s s the "S" bend of the F r a s e r R i v e r to the Camelsfoot Range, F i g . 3.2 ( D u f f e l l and McTaggart, 1952). These v o l c a n i c and s edimentary r o c k s appear to be h i g h l y susoe.p.tible to w e a t h e r i n g and thus c o n t r a s t s t r o n g l y w i t h the g r a n i t i c r o c k s on the western bank of the F r a s e r . C o n s e q u e n t l y , h i l l s i d e s u n d e r l a i n by the former are s e v e r e l y g u l l i e d and s c a r r e d by numerous t a l u s streams and " s l i d e a r e a s " . Hydrothermal a l t e r a t i o n i n some of the v o l c a n i c s has g i v e n r i s e to prominent m u l t i - c o l o u r e d f o r m a t i o n s w i t h a deep and e a s i l y eroded s u r f a c e l a y e r . The Mount L y t t o n b a t h o l i t h , c o n s i s t i n g p r e d o m i n a n t l y of d i o r i t e and bounded by f a u l t s , u n d e r l i e s the h i g h l a n d area around B o t a n i e Mountain at the s o u t h e r n end of the C l e a r Range, and extends northwards t o the c o n f l u e n c e of the F r a s e r w i t h C i n q u e f o i l 43 Creek, F i g . 3 . 2 . In d e t a i l , the F r a s e r R i v e r f a u l t zone c o n s i s t s of a s e r i e s of f a u l t s t r e n d i n g from n o r t h - n o r t h w e s t to s o u t h -s o u t h e a s t . The zone i s o c c u p i e d by a b e l t of C r e t a c e o u s r o c k s ( J a c k a s s Mountain and L i l l o o e t groups) v a r y i n g i n w i d t h from a few hundred yard s to 6 m i l e s , F i g . 3.2. I t has the form of a graben between C i n q u e f o i l Creek and L y t t o n , but i n the more n o r t h e r l y s e c t i o n i t i s i n t e r m e d i a t e ( i n terms of r e l a t i v e e l e v a t i o n ) between areas to the e a s t and west. The most r e c e n t l y proven movement a l o n g the f a u l t s i s post-Eocene but not l a t e r than P l i o c e n e ( D u f f e l l and McTaggart, 1952), thus a l t h o u g h the f a u l t p l a n e s appear to c o n t r o l the form of the v a l l e y s i d e s i n s e v e r a l p l a c e s (as f a u l t - l i n e s c a r p s ) , no movement has a f f e c t e d the d i s p o s i t i o n of Quaternary d e p o s i t s or l a n d f o r m s . I t was suggested by T r e t t i n (>1960 ) t h a t the F r a s e r R i v e r p r e v i o u s l y f o l l o w e d the e a s t e r n boundary f a u l t through the Three Lake V a l l e y c o n n e c t i n g Fountain.' and C i n q u e f o i l C r e e k s , thus b y - p a s s i n g the "S" bend between F o u n t a i n and L i l l o o e t . The F r a s e r was l a t e r d i v e r t e d to i t s p r e s e n t course presumably due to c a p t u r e by the B r i d g e R i v e r which o r i g i n a l l y o c c u p i e d the L i l l o o e t s e c t i o n of the p r e s e n t F r a s e r V a l l e y . The date of. the supposed c a p t u r e i s u n c e r t a i n , but i t must have taken p l a c e l o n g b e f o r e g l a c i a t i o n , p o s s i b l y as a r e s u l t of the P l i o c e n e u p l i f t , or more p r o b a b l y e a r l i e r as a r e s u l t of the l a t e - C r e t a c e o u s u p l i f t and subsequent e r o s i o n ( c f . 1.1.1). The p r e s e n t e l e v a t i o n of the F o u n t a i n -44 C i n q u e f o i l c o l i s 3,250', w h i l e the p r e s e n t e l e v a t i o n of the F r a s e r R i v e r at L i l l o o e t i s 640'. The h i g h e s t p o s t -g l a c i a l t e r r a c e i n the F o u n t a i n r e - e n t r a n t of the F r a s e r V a l l e y stands at 1,320'. Thus at p r e s e n t , the F r a s e r i s 2 5/8/00' below the proposed former c o u r s e , and the e a r l i e s t p o s t - g l a c i a l r i v e r was about 1,500' below the c o l . Thus i t i s l i k e l y t h a t the c a p t u r e took p l a c e at a c o n s i d e r a b l e . t i m e b e f o r e the g l a c i a t i o n . The p r e s e n t morphology of the F r a s e r V a l l e y i s , to a g r e a t e x t e n t , the r e s u l t of i t s p o s t - g l a c i a l h i s t o r y . D u r i n g the phase of a g g r a d a t i o n t h a t commenced d u r i n g the m e l t i n g of the P l e i s t o c e n e i c e , the bedrock f l o o r o f the v a l l e y was b u r i e d by up to 1,000' of s i l t , sand and g r a v e l . The s t r a t i -graphy of t h e s e d e p o s i t s i n d i c a t e s a complex s e r i e s of events d u r i n g t h i s t i m e . L a t e r d i s s e c t i o n by the F r a s e r l e f t t h i s a g g r a d a t i o n a l s u r f a c e s t a n d i n g as a bench s e v e r a l hundred f e e t above r i v e r l e v e l . The sequence of events was not s i m i l a r f o r a l l p a r t s of the F r a s e r V a l l e y , c o n s e q u e n t l y , a t h r e e - . f o l d d i v i s i o n of the area was made on t h i s b a s i s . Between P a v i l i o n and the B r i d g e R i v e r , s e d i m e n t a t i o n i n the F r a s e r V a l l e y i s c h a r a c t e r i z e d by t h i c k sequences of f l u v i a l , or f l u v i o - g l a c i a l m a t e r i a l ; between Texas Creek and the B r i d g e R i v e r the c h i e f s edimentary u n i t i s a g l a c i o - l a c u s t r i n e s i l t ; between Texas Creek and L y t t o n f l u v i o - g l a c i a l g r a v e l s and mudflow g r a v e l s are predominant. 45 3 . 2 The F r a s e r V a l l e y between Texas Creek and the B r i d g e  R i v e r The g l a c i o - l a c u s t r i n e s i l t t h a t c h a r a c t e r i z e s t h i s r e g i o n was f i r s t noted and r e c o r d e d by D u f f e l l and McTaggart (1952), but no d e t a i l e d i n t e r p r e t a t i o n has been attempted p r e v i o u s l y . The s i l t i s seen at i n t e r v a l s a l o n g the cut banks of the F r a s e r between a p o i n t a p p r o x i m a t e l y o n e - h a l f m i l e upstream from the F r a s e r - S e t o n c o n f l u e n c e t o a l o c a t i o n one m i l e south of the mouth of Texas Creek, F i g . 3.3. The s i l t i s b e l i e v e d t o have been d e p o s i t e d i n a g l a c i a l l a k e , r e f e r r e d to h e n c e f o r t h as G l a c i a l Lake L i l l o o e t , s i n c e i t o c c u p i e d the p r e s e n t s i t e of t h a t town. The s i l t w i l l be r e f e r r e d t o as the L i l l o o e t s i l t , and by mode of o r i g i n , i t bears comparison w i t h o t h e r l a t e - g l a c i a l s i l t d e p o s i t s t h a t occupy many of the v a l l e y s of the I n t e r i o r P l a t e a u , such as the South Thompson S i l t ( F u l t o n , 1965) and the " w h i l e s i l t " of the Okanagan ( F l i n t , 1935). 3.2.1 The L i l l o o e t S i l t (1) S t r a t i g r a p h y and C o m p o s i t i o n The s i l t i s s t r a t i g r a p h i c a l l y t he lowe s t of the Quaternary d e p o s i t s exposed i n t h i s a r e a . I t appears to be v a r i a b l e w i t h r e g a r d t o c h a r a c t e r i s t i c s such as bedding and c o l o u r . The bedding v a r i e s from p l a c e t o p l a c e a c c o r d i n g to the l o c a l c o n d i t i o n s of d e p o s i t i o n , d r y i n g and t e x t u r e . Where i t i s w e l l bedded, the beds are t h i n , r a n g i n g up t o 2", but commonly, the d e p o s i t appears massive w i t h w e a t h e r i n g and 47 d r y i n g a t the s u r f a c e g i v i n g r i s e t o a b l o c k y t e x t u r e . The c o l o u r of the s i l t ranges from l i g h t - g r e y t o cream, depending upon the s t a t e of w e a t h e r i n g and the amount of water c o n t a i n e d . I t i s b l u e i n u n o x i d i z e d s e c t i o n s . Samples of the L i l l o o e t S i l t were c o l l e c t e d from f i v e exposures between L i l l o o e t and Texas Creek (samples S19, S23, S24, S25 and S26, Appendix #1). Mean g r a i n s i z e i s +6.490 ( e q u i v a l e n t to 0.0112 mm.); the average c o m p o s i t i o n i s 8.8% sand, 70.4% s i l t , 20.8% c l a y ; s o r t i n g i s poor (C 0 = - 2 . 0 9 ) . In p l a c e s the s i l t i s i n t e r b e d d e d w i t h l e n s e s or l a y e r s of f i n e sand; mudflow m a t e r i a l i s a l s o f a i r l y common w i t h i n the s i l t sequence i n the form of l e n s e s t h a t are nowhere c o n t i n u o u s over d i s t a n c e s of more than about 30'. These c o a r s e r m a t e r i a l s p r o b a b l y r e p r e s e n t d e p o s i t i o n d u r i n g p e r i o d s of h i g h stream d i s c h a r g e i n t o the l a k e , or l o c a l d e p o s i t i o n from g u l l i e s and h i l l s i d e s i n the case of the mudflows. I s o l a t e d p e b b l e s and c o b b l e s were f r e q u e n t l y found i n the s i l t and are b e l i e v e d to have been c a r r i e d by i c e - r a f t i n g a c r o s s the g l a c i a l l a k e . In many p l a c e s the s i l t has been d i s t u r b e d by slumping d u r i n g i t s d e p o s i t i o n . E vidence of t h i s i s f r e q u e n t l y seen as c o n t o r t e d bedding of v a r i o u s forms and s i z e s . The l a r g e s t c o n t o r t i o n s are w e l l exposed i n the bank of the F r a s e r o p p o s i t e the mouth of Seton R i v e r , F i g . 3.4. T'nfevy' occur i n t h i n l y bedded f i n e grey s i l t and resemble overfold.s w i t h beds s t r e t c h e d and i n p l a c e s almost broken between F i g u r e 3.4. The L i l l o o e t S i l t : s i l t w i t h con-t o r t i o n s due to slumping o v e r l a i n unconform-a b l e by t e r r a c e - c a p p i n g of c o b b l e - g r a v e l ; . e a s t e r n bank of F r a s e r R i v e r at c o n f l u e n c e w i t h Seton R i v e r . F i g u r e 3.5. The L i l l o o e t S i l t : slump b a l l s i n s i l t on f r o n t a l s l o p e of Seton D e l t a ; spade l e n g t h 2 f e e t . 49 i n d i v i d u a l f o l d s . A p p a r e n t l y , the d i r e c t i o n of movement was d o w n v a l l e y . The zone of c o n t o r t e d bedding i s v e r y l o c a l i z e d , b e i n g no more than 30' i n t h i c k n e s s and 90' i n w i d t h . A d j a c e n t s i l t beds show o n l y v e r y s m a l l a n g l e s of d i p , s u g g e s t i n g t h a t the c o n t o r t i o n s r e s u l t e d from v e r y l o c a l i z e d slumping which took p l a c e d u r i n g d e p o s i t i o n . Much s m a l l e r c o n t o r t i o n s were found exposed i n a r a i l -r o a d c u t t i n g one m i l e t o the southwest, of L i l l o o e t . These s t r u c t u r e s c o n s i s t of " w h o r l s " of l a m i n a t e d s i l t and sand, f r e q u e n t l y w i t h a sand c o r e , F i g . 3.5. Each w h o r l i s u s u a l l y l e s s than 2' i n d i a m e t e r . The beds were crumpled d u r i n g the r o l l i n g - u p p r o c e s s to produce a p s e u d o - r i p p l e m a r k s t r u c t u r e . P o s s i b l y t h i s slumping took p l a c e down the f r o n t a l s l o p e of a d e l t a b e i n g b u i l t by the Seton R i v e r i n t o G l a c i a l Lake L i l l o o e t ( c f . 3.2.2 f o l l o w i n g ) . S i m i l a r f e a t u r e s are com-mon i n the s i l t elsewhere i n t h i s a r e a . L a r g e r c o n t o r t i o n s , but w i t h s i m i l a r s t r u c t u r e , are d e s c r i b e d i n d e t a i l from the Thompson V a l l e y ( c f . 4.2.1 f o l l o w i n g ) . (2) T h i c k n e s s The maximum e l e v a t i o n of the upper s u r f a c e of the s i l t i s 1,000' a . s . l . where i t i s found beneath the Texas Creek and Jones Ranch benches. S i n c e the upper s u r f a c e of the s i l t i s e r o s i o n a l wherever i t was found ( w i t h one excep-t i o n ) , 1,000' a . s . l V x i s a minimum v a l u e f o r the e l e v a t i o n of the s u r f a c e of the- l a k e i n which the s i l t was d e p o s i t e d . V1' /' The e x c e p t i o n a l case of-'-'a conformable c o n t a c t a t the upper s u r f a c e of the s i l t was found i n the r a i l r o a d exposure one 50 m i l e t o the southwest of L i l l o o e t . Here the s i l t grades upwards, i n t o a d e l t a i c complex of sands and g r a v e l s ( c f . 3.2.2), but the s u r f a c e of t h i s d e l t a i s below 1,000' and hence g i v e s no c l o s e r e s t i m a t e of the l e v e l of the l a k e s u r f a c e. Where exposures are not obscured at lower e l e v a t i o n s by r e c e n t s l u m p i n g , the s i l t i s seen to c o n t i n u e down to r i v e r l e v e l . Some i n d i c a t i o n of i t s f u r t h e r e x t e n t i s i n d i -c a t e d i n a w e l l l o g f o r a p o i n t o n e - h a l f m i l e south of the S e t o n - F r a s e r c o n f l u e n c e on a low, east-bank t e r r a c e . The w e l l l o g was i n t e r p r e t e d as f o l l o w s : Top of s e c t i o n T h i c k n e s s I n t e r p r e t a t i o n ( f e e t ) Sand, s i l t 13 ) d e g r a d a t i o n a l t e r r a c e B o u l d e r s , c o b b l e s , g r a v e l 21 ; ca p p i n g S i l t 198 S i l t y g r a v e l 19 T i l l ( ? ) 5 ) t i l l ( ?) L i l l o o e t s i l t The s u r f a c e e l e v a t i o n of the t e r r a c e where t h i s w e l l i s l o c a t e d i s 680'. Thus the s i l t extends downwards to 448' a . s . l . and assuming t h a t the s u r f a c e e l e v a t i o n of the s i l t i s h o r i z o n t a l (and at 1,000') then the i n d i c a t e d t h i c k -ness i s 552'. The maximum observed t h i c k n e s s of the s i l t i s a p p r o x i m a t e l y 300' below a l a r g e bench on the s o u t h e r n s i d e of the mouth of Texas Creek. (3) Upper c o n t a c t The s i l t i s g e n e r a l l y o v e r l a i n unconformably by e i t h e r c o b b l e g r a v e l s or mudflow d e p o s i t s t h a t were l a i d down d u r i n g the s u b - a e r i a l c o n d i t i o n s t h a t succeeded the l a k e phase. Beneath th e s e two f o r m a t i o n s , the upper s u r f a c e 51 of the s i l t d i s p l a y s numerous c u t - a n d - f i l l f e a t u r e s , F i g . 3.6. 3.2.2 The Seton D e l t a At the mouth of the Seton R i v e r the L i l l o o e t s i l t i s conformably o v e r l a i n by a d e l t a i c sequence (as mentioned above). T h i s comprises one of the main sedimentary u n i t s i n the a r e a . The d e l t a o r i g i n a l l y o c c u p i e d the e n t i r e r e - e n t r a n t area at the mouth of the Seton R i v e r , but a l a r g e volume of i t s m a t e r i a l has s i n c e been removed by t h i s r i v e r as i t ca r v e d a s e r i e s of d e g r a d a t i o n a l t e r r a c e s here. Good exposures i n c u t t i n g s a l o n g the P.G.E. r a i l r o a d t r a c k s show a t y p i c a l d e l t a i c s e r i e s of s i l t , sand and g r a v e l : Exposure 1 T h i c k n e s s ( f e e t ) L o c a t i o n : 1\ m i l e s west of L i l l o o e t P.G.E. s t a t i o n C o m p o s i t i o n : f i n e l y i n t e r b e d d e d y e l l o w s i l t and grey sand w i t h occa-s i o n a l t h i n l e n s e s of f i n e , rounded g r a v e l up to 18" t h i c k ; the upper-most bed i s a co a r s e c o b b l e g r a v e l , n o n - s o r t e d and non-bedded; i t grades eastwards i n t o s i l t and sand; F i g . 3.7. S t r u c t u r e : d i p s toward the east r a n g i n g from 2° to 30°; mean d i p i s 8°. Minor s t r u c t u r e s i n c l u d e s m a l l l o a d c a s t s , c o n t o r t i o n s i n a few d i s -t i n c t i v e beds, s m a l l normal f a u l t s w i t h downthrow of 2" or 3" to e a s t . 50' t o 60' Exposure 2. L o c a t i o n : 1 to 1 1/4 m i l e s west of L i l l o o e t P.G.E. S t a t i o n C o m p o s i t i o n : massive s i l t (approx. 3 0 ' ) , o v e r l y i n g a bedded s e r i e s of s i l t and sand which i n c l u d e s (1) beds F i g u r e 3 . 6 . The L i l l o o e t S i l t : c h a nnel i n -f i l l i n g at c o n t a c t of s i l t w i t h t e r r a c e -c apping of g r a v e l , sand and a e o l i a n s i l t . F i g u r e 3 . 7 . Seton D e l t a : Exposure #1 f o r e s e t s of i n t e r b e d d e d s i l t and f i n e w i t h some g r a v e l l e n s e s . showing sand 53 T h i c k n e s s ( f e e t ) of c o n t o r t e d s i l t w i t h sand i n c l u -s i o n s ; o r i g i n a l bedding d e s t r o y e d by s l u m p i n g ; (2) beds of i n t e r l a m i n -a t e d f i n e grey sand and s i l t ; s i l t has same g r a i n - s i z e c o m p o s i t i o n as the 60' L i l l o o e t S i l t (see Appendix #1 f o r d e t a i l s ) ; (3) two prominent beds of sand w i t h l o a d s t r u c t u r e s ; up to 18" t h i c k . S t r u c t u r e : bedding i s h o r i z o n t a l w i t h s m a l l l o c a l u n d u l a t i o n s . Exposure 3. L o c a t i o n : 3/4 m i l e s west of L i l l o o e t P.G.E. S t a t i o n C o m p o s i t i o n : a l t e r n a t i n g beds of c o n t o r t e d and n o n - c o n t o r t e d s i l t ; upper boundary of s i l t i s e r o s i o n a l 50' c o n t a c t w i t h t e r r a c e capping of c o b b l e g r a v e l . S t r u c t u r e : d i p a p p r o x i m a t e l y 5° t o e a s t . The whole sequence i s i n t e r p r e t e d as a d e l t a t h a t was b u i l t i n t o G l a c i a l . L a k e L i l l o o e t by the l a t e - g l a c i a l Seton R i v e r . The g e n t l y d i p p i n g sand and s i l t beds, becoming f i n e r e a s t w a r d s , r e p r e s e n t the f o r e s e t s ; the f r e -quent c o n t o r t i o n s i n the s i l t beds i n d i c a t e t h a t slumping down the f o r e s e t s l o p e was a common o c c u r r e n c e . A l s o , s i n c e s i l t c o n t o r t i o n s occur w i t h i n beds t h a t appear h o r i z o n t a l , i t seems t h a t e i t h e r a very low s l o p e was s u f f i -c i e n t to p e r m i t slumping to o c c u r , or the slumps moved out beyond the s l o p e t h a t s e t them i n motion to a f l a t t e r a r ea of the l a k e f l o o r . The conformable c o a r s e r m a t e r i a l above the s a n d - s i l t sequence i n Exposure 1 i s b e l i e v e d t o be a remnant of the o r i g i n a l t o p s e t beds, but most of these have been eroded and 54 r e p l a c e d by t e r r a c e cappings of c o b b l e g r a v e l s , as i n Exposure 3, at some l a t e r d a t e . The uppermost d e l t a i c sediments are at an e l e v a t i o n of s l i g h t l y more than 880', so the l a k e s u r f a c e must have stood a t l e a s t t h i s h i g h d u r i n g depos i t ion--and perhaps con-s i d e r a b l y h i g h e r - - s i n c e the o r i g i n a l t o p s e t beds have been m o s t l y removed by e r o s i o n . 3.2.3 The F o u n t a i n D e l t a To the n o r t h of the F r a s e r - S e t o n c o n f l u e n c e the s i l t d i s a p p e a r s and i s r e p l a c e d by a t h i c k s e r i e s of g r a v e l s . T h i s change t a k e s p l a c e over a d i s t a n c e of 3/4- m i l e between the mouth of Seton R i v e r and the L i l l o o e t v i e w p o i n t , but t h e r e are no exposures w i t h i n t h i s a rea to i n d i c a t e i f the t r a n s i t i o n from s i l t t o g r a v e l i s abrupt or g r a d a t i o n a l . The g r a v e l s extend upstream beyond the mouth of the B r i d g e R i v e r and are a s s o c i a t e d w i t h a s e r i e s of kame d e p o s i t s near F o u n t a i n S t a t i o n . These are d i s c u s s e d i n S e c t i o n 3.3.1 f o l l o w -i n g . The n a t u r e of the g r a v e l s i n the F o u n t a i n D e l t a i s w e l l exposed i n the r a i l r o a d c u t t i n g t o the n o r t h of L i l l o o e t between the s a w m i l l on the n o r t h e r n t i p of the t o w n s i t e and the P.G.E. b r i d g e a c r o s s the F r a s e r . The sequence of g r a v e l s observed at f i v e l o c a t i o n s a l o n g the exposure i s i n d i c a t e d i n F i g . 3.8. The g r a v e l s are c h a r a c t e r i s t i c a l l y p o o r l y sorted,, p o o r l y bedded and of c o a r s e t e x t u r e ; l a r g e s c a l e c r o s s - b e d d i n g i s common w i t h h i g h l y v a r i a b l e d i r e c t i o n s SECTIONS WITHIN THE FOUNTAIN DELTA ft. a. s.l. 9 6 0 -950 -9 4 0 -930 -9 2 0 -910 -south north LEGEND E O £> O o O • 0 ooo ft. - terrace capping - cobble gravel - bedded gravel or sand - cross-bedded gravel or sand - gravel lense • ••• • ••• • • * • * 4 o ' o ' - ' 3 •••• ' 0 o 0 ' <j '0 OOP - •W *?•:?•. • / V •o-p-o.o: 0 O \y~oo ^ - '0- ?.?Tj! o.'o0-• • o. o-, < . 0 -.0 a ... o w Q •0 ° •6 °. <§>°° % = § ^ > ? '.'."0 0 • '• 0 CO o »? -o" o . 0 <v? :£>»-• -° A * - © o o e o n o ° " o - - D • - . .v i : ' /?" well - sorted material poorly-sorted material sand silt till or mudflow gravel 77 ft* •OO-O-O • 6 0 0 0 ' . A - O - O -boVo'' •OOGoO 0«0o0 OCOo - • ooo oopo-;*;,".>»• •; 0; • A7-!£ O.Ot .. o"- -• oOoO--gOoi> b6»oO Oa'qoo-eoooe' c m CO 56 and a n g l e s of d i p . F i n e m a t e r i a l such as sand and s i l t i s r e l a t i v e l y r a r e at t h i s l o c a t i o n , b e i n g d e p o s i t e d f u r t h e r out i n the g l a c i a l l a k e . An u n u s u a l g r a v e l l a y e r o c c u r s low i n the sequence i n S e c t i o n s 2, 3, 5 and 6. ( F i g u r e 3.8, " f i l l or mudflow g r a v e l " ) . I t i s the most c o n t i n u o u s and l e a s t v a r i a b l e bed w i t h i n the e n t i r e g r a v e l sequence h e r e , and c o n s i s t s of w e l l i n d u r a t e d m a t e r i a l t h a t i s a p p r o x i m a t e l y 50% g r a v e l and 50% f i n e g r a i n e d m a t r i x ( c l a y , s i l t and sand). I t i s between 2.5' and 3' i n t h i c k n e s s w i t h the upper 4" b e i n g b r i g h t r e d . From i t s t e x t u r e and c o m p o s i t i o n i t c o u l d be i n t e r p r e t e d e i t h e r as a mudflow or as a t h i n t i l l s h eet. As a mudflow i t may have o r i g i n a t e d as a f l o w of water s a t u r a t e d m a t e r i a l from e i t h e r the v a l l e y s i d e s or from o f f the snout of the i c e i t s e l f ; as a t i l l sheet i t would r e p r e s e n t a s l i g h t readvance of the i c e f r o n t . In e i t h e r of t h e s e r a t h e r s i m i l a r s i t u a t i o n s , the r e d l a y e r , i f caused by w e a t h e r i n g would i m p l y t h a t f o r some time a f t e r i t s d e p o s i -t i o n t h i s bed formed the ground s u r f a c e and was exposed to the e f f e c t s of weather b e f o r e b e i n g b u r i e d by the l a k e , and l a t e r , the l a k e s ediments. In t h i s r e s p e c t a low l e v e l l a k e s t a g e , w i t h the s u r f a c e below 915', would have been f o l l o w e d by a r i s e i n l a k e l e v e l to at l e a s t 960' . A l t e r n a t e l y , the r e d l a y e r may have r e s u l t e d from d e p o s i -t i o n of o r i g i n a l l y r e d m a t e r i a l , which may be s i m i l a r t o i n c l u s i o n s of r e d T e r t i a r y conglomerate found w i t h i n t i l l between F o u n t a i n and P a v i l i o n ( c f . 3.3.1 f o l l o w i n g ) . 57 The g r a v e l s of the F o u n t a i n D e l t a l i e beneath a d e g r a d a t i o n a l r i v e r t e r r a c e w i t h a s u r f a c e at 960'. Immediately beneath the t e r r a c e s u r f a c e i s the t y p i c a l t e r r a c e c a p p i n g of c o b b l e - g r a v e l . Thus here 960' i s the minimum p o s s i b l e e l e v a t i o n f o r the l a k e s u r f a c e . The e n t i r e g r a v e l sequence (except the uppermost c o b b l e - g r a v e l ) i s i n t e r p r e t e d as h a v i n g been d e p o s i t e d i n c l o s e p r o x i m i t y to an i c e f r o n t . W i t h i n the g r a v e l s are s e v e r a l s u b s i d e n c e or k e t t l e s t r u c t u r e s , i n d i c a t i n g b u r i a l of l a r g e i c e b l o c k s d u r i n g d e p o s i t i o n . The c h a o t i c g r a v e l beds t h a t are w i t h o u t e i t h e r s o r t i n g or bedding i n d i c a t e v e r y r a p i d d e p o s i t i o n - - p o s s i b l y almost i n c o n t a c t w i t h the i c e f r o n t - - a n d so t h i s g r a v e l may w e l l be c l a s s e d as a form of water l a i d moraine. The absence of f i n e s would suggest t h a t s w i f t l y f l o w i n g streams of water c r o s s e d the a r e a , c a u s i n g the sand and s i l t t o be c a r r i e d on and d e p o s i t e d f u r t h e r out i n t o the l a k e . Rapid d e p o s i t i o n i n a l a c u s t r i n e environment i s a l s o i n d i c a t e d by the l a r g e s c a l e c r o s s b e d d i n g , see F i g s . 3.9 t o 3.12. A sequence of g r a v e l s s i m i l a r t o t h a t d e s c r i b e d above i s exposed on the e a s t e r n bank of the F r a s e r between the L i l l o o e t v i e w p o i n t and the road b r i d g e . Here, a t h i n l a y e r of a e o l i a n sand o v e r l i e s t e r r a c e c o b b l e - g r a v e l s which i n ' t u r n unconformably o v e r l i e a sequence of c o a r s e g r a v e l w i t h s o r t -i n g and bedding r a n g i n g from moderate to n o n - e x i s t a n t . A ngular b l o c k s d e r i v e d from nearby bedrock are a l s o i n c l u d e d . F i g u r e 3.9. F o u n t a i n D e l t a : i n f i l l e d k e t t l e h o l e . F i g u r e 3.10. F o u n t a i n D e l t a : l a r g e - s c a l e c r o s s - b e d d i n g . F i g u r e 3.11. F o u n t a i n D e l t a r e l a t i v e l y f i n e f o r e s e t g r a v e l s above " c h a o t i c " g r a v e l s . F i g u r e 3.12. F o u n t a i n D e l t a : exposure i l l u s -t r a t i n g v a r i a t i o n i n p a r t i c l e s i z e s , s o r t i n g and bedding; bed of t i l l - l i k e m a t e r i a l at base b e s i d e f i g u r e . 60 3.2.4 The R i v e r l a n d s Kame T e r r a c e R i v e r l a n d s . t e r r a c e i s another i m p o r t a n t l a n d f o r m and sedimentary u n i t t h a t o r i g i n a t e d d u r i n g the d e g l a c i a t i o n of the L i l l o o e t a r e a . I t stands as a prominent t e r r a c e on the e a s t e r n bank of the F r a s e r t o the s o u t h e a s t of L i l l o o e t , F i g . 3.3. The t e r r a c e s u r f a c e i s u n d u l a t i n g w i t h a l o c a l r e l i e f of about 200' and i n c l u d e s s e v e r a l c l o s e d d e p r e s s i o n s . L i t t l e i s known of the i n t e r n a l s t r u c t u r e of t h i s f e a t u r e , s i n c e no l a r g e exposures e x i s t , however, two s m a l l g r a v e l p i t s i n the scarp' s l o p e r e v e a l s t e e p l y d i p p i n g , m o d e r a t e l y s o r t e d beds of rounded g r a v e l . Rounded peb b l e s are found everywhere on the s u r f a c e of t h i s t e r r a c e . A s h a l l o w c h a n n e l s e p a r a t e s the feature, from the h i l l s i d e t o the e a s t . T h i s l a n d f o r m i s i n t e r p r e t e d as a kame t e r r a c e . I t i s not a m a r g i n a l f e a t u r e , s i n c e i t s w i d t h (1 1/4 m i l e s ) i s over h a l f the w i d t h of the whole v a l l e y at t h i s p o i n t . R a t h e r , i t i s b e l i e v e d t o have been c o n s t r u c t e d between two or more major v a l l e y l o b e s of i c e d u r i n g the v a l l e y g l a c i e r phase of d e g l a c i a t i o n . T h i s area would then be the f i r s t i c e - f r e e ground on the v a l l e y f l o o r . Steep i c e c o n t a c t s l o p e s are found a l o n g the n o r t h e r n and s o u t h e r n edges of the kame, and the h i g h e s t p a r t s of the t e r r a c e are a l s o a l o n g t h e s e r i m s . A lower t e r r a c e , p o s s i b l y kame, b o r d e r s the major t e r r a c e t o the n o r t h , and may have been con-s t r u c t e d a g a i n s t the n o r t h e r l y i c e lob e at a s l i g h t l y l a t e r date ( F i g . 3.3). 61 Other s m a l l e r " l a t e r a l " kame t e r r a c e s , formed by d e p o s i t i o n a l o n g the margin of the d e c a y i n g v a l l e y g l a c i e r , were p o s s i b l y formed at about the same time as the R i v e r l a n d s kame t e r r a c e . S e v e r a l of t h e s e are to be found a l o n g the v a l l e y s i d e s upstream from L i l l o o e t , F i g . 3.3. 3.2.5 •Bedrock Gorges a l o n g the F r a s e r Other f e a t u r e s of importance w i t h r e g a r d t o the p o s t - g l a c i a l h i s t o r y of the v a l l e y are bedrock gorges a l o n g the F r a s e r R i v e r i n at l e a s t two l o c a t i o n s . The gorges are b e l i e v e d to have been e x c a v a t e d i n p o s t - g l a c i a l time by the F r a s e r R i v e r , so t h a t i t now o c c u p i e s a c h a n n e l w i t h bedrock w a l l s over 200' h i g h i n these l o c a t i o n s . The more u s u a l s i t u a t i o n i s t h a t the F r a s e r has cut p o s t - g l a c i a l channels c h i e f l y i n u n c o n s o l i d a t e d d e p o s i t s . Between 3^ and m i l e s south of the F r a s e r - S e t o n c o n f l u e n c e , the F r a s e r f l o w s i n a bedrock gorge around the edge of the bench upon which the Jones Ranch i s s i t u a t e d , F i g . 3.3. The bedrock gorge i s up to 500' deep. To the n o r t h and south of the bedrock zone w h i t e s i l t i s exposed, i n the r i v e r banks. The second bedrock gorge i s l o c a t e d to the n o r t h of the mouth of Texas Creek, F i g . 3.3. Once a g a i n the r i v e r f l o w s i n a gorge a g a i n s t the e a s t e r n s i d e of the v a l l e y , w h i l s t a s u s p e c t e d b u r i e d c hannel runs a l o n g the western s i d e . • There are two p o s s i b l e s i t u a t i o n s which c o u l d have l e d to the c u t t i n g of these gorges: g l a c i a l e r o s i o n may have 62 formed an i r r e g u l a r bedrock f l o o r f o r the v a l l e y c o n s i s t i n g . of a s e r i e s of deep b a s i n s s e p a r a t e d by h i g h e r r o c k steps:, a l t e r n a t e l y , the p o s t - g l a c i a l r i v e r d i d not re-occupy i t s pre- ( o r i n t e r - ) g l a c i a l channel', but carved a new one at the s i d e o f , and p a r a l l e l t o , the e a r l i e r one. I f the l a t t e r s i t u a t i o n h o l d s , then the o l d e r channels must e x i s t , b u r i e d by g l a c i a l or p o s t - g l a c i a l d e p o s i t s , and the d i v e r s i o n of the r i v e r must have been due to b l o c k a g e by i c e or g l a c i a l d e b r i s . The f i r s t p o s s i b i 1 i t y - - o f d i f f e r e n t i a l i c e e r o s i o n -appears u n l i k e l y . No s i m i l a r p a t t e r n of g l a c i a l e r o s i o n has been observed elsewhere a l o n g the v a l l e y s of the I n t e r i o r P l a t e a u . W i t h i n the F r a s e r V a l l e y , the maximum topography of the bedrock f l o o r i s l e s s than 150' and was observed a l o n g the s e c t i o n of the r i v e r between P a v i l i o n and F o u n t a i n . A l s o , d i f f e r e n t i a l e r o s i o n , of the o r d e r of 500' d i f f e r e n c e s i n e l e v a t i o n of the v a l l e y f l o o r over a d i s t a n c e of a m i l e or l e s s , would r e q u i r e e r o s i o n to be much s t r o n g e r than i s g e n e r a l l y supposed to have been the case on the f l o o r of a v a l l e y t r e n d i n g a c r o s s the g e n e r a l d i r e c t i o n of i c e movement and b u r i e d t o a depth of 7,000' to 8,000' ( c f . 2.1.1). A b e t t e r case can be made f o r a p o s t - g l a c i a l d i v e r s i o n of the r i v e r . There, i s c o n s i d e r a b l e e vidence to i n d i c a t e t h a t the b u r i e d former r i v e r c h a n n els do e x i s t f o r both the Jones and the Texas gorges. To the n o r t h and south of the Jones gorge s i l t t h a t i s exposed i n the r i v e r banks i s b e i n g 63 eroded by s m a l l g u l l i e s and the downstream p a r t of Wick Creek. These g u l l i e s do not t r e n d at r i g h t a n g l e s t o the r i v e r , but are d i v e r t e d t o t r e n d d i a g o n a l l y a c r o s s the n o r t h e r n and s o u t h e r n margins of the Jones bench, as i f f o l l o w i n g a s-i'l't" i r i ' f ' i l 1.ing; a l o n g the western, margin of the bench, see F i g . 3.3. T h i s suggests t h a t a body of uncon-s o l i d a t e d m a t e r i a l may e x i s t h e r e , e x t e n d i n g a l o n g the western edge of the v a l l e y f l o o r and s e p a r a t e d by bedrock from the p r e s e n t r i v e r c h a n n e l . The topography of the s u r f a c e of the Jones bench, would a l s o suggest d i v e r s i o n of the r i v e r ; the h i g h e s t p a r t of the bench i s l o c a t e d a l o n g i t s e a s t e r n edge, a d j a c e n t t o the p r e s e n t r i v e r i n the gorge, whereas the l o w e s t l y i n g a r e a i s a d j a c e n t t o the western v a l l e y s i d e , above the supposed b u r i e d c h a n n e l , w i t h a d i f f e r e n c e i n e l e v a t i o n between the two areas of about 100 ' . There i s more d i r e c t e v idence f o r s i l t i n f i l l i n g of a. channel beneath the bench at the mouth of Texas Creek. The bench here has the morphology of a d i s s e c t e d a l l u v i a l f a n w i t h good exposures beneath the s u r f a c e of the f a n a l o n g the steep w a l l s of the d i s s e c t i o n gully.:;-." Beneath the most e a s t e r l y p a r t of the " f a n " , bedrock i s exposed i n both the n o r t h and south s i d e s of the g u l l y / ; e x t e n d i n g the e n t i r e t h i c k n e s s of the " f a n " , from g u l l y ' f l o o r to s u r f a c e . The bedrock i s c o n t i n u o u s a l o n g the n o r t h e r n s i d e of the gully,'/ as f a r as the exposure goes, t h a t i s , about t w o - t h i r d s of the way to the f a n apex. On the southern s i d e of the g u l l y , 64 the bedrock i s o n l y found beneath the o u t e r t i p of the-bench, the r e m a i n i n g p a r t of the g u l l y w a l l c o n s i s t i n g c h i e f l y of s i l t . The i n f e r r e d p o s i t i o n of the b u r i e d c h a n n e l i s i n d i -c a t e d on F i g . 3.3. U n f o r t u n a t e l y , t h e r e are no r e c o r d s of any w e l l s d r i l l e d beneath e i t h e r of these benches, so a l t h o u g h i t i s apparent t h a t s i l t i n f i l l s the o l d c h a n n e l i n the case of the Texas d i v e r s i o n , the n a t u r e of the i n f i l l i n g at Jones bench i s not known. I t may p o s s i b l y , c o n s i s t of t i l l or some ot h e r k i n d of g l a c i a l d e p o s i t . I f so t h i s would n e c e s s i -t a t e no more than one phase of i c e melt f o r the f o r m a t i o n of the bedrock gorge, whereas an i n f i l l i n g of s i l t supposes the v a l l e y was i c e f r e e w h i l s t s i l t d e p o s i t i o n took p l a c e , and then a l a t e r r e t u r n of i c e l e d to the d i v e r s i o n . Supposing the Jones b u r i e d c h a n n e l to be i n f i l l e d w i t h t i l l , then i t i s q u i t e p o s s i b l e t h a t d u r i n g the f i n a l stages i n the m e l t i n g of the v a l l e y i c e , the i c e s u r f a c e s l o p e d from west t o east a c r o s s the v a l l e y . That i s , a s l o p e away from the source of the i c e i n the l o c a l peaks of the Coast Range. Thus the e a r l y p o s t - g l a c i a l F r a s e r R i v e r would have been c o n f i n e d a g a i n s t the e a s t e r n v a l l e y w a l l at the time i t was f u n c t i o n i n g as a s p i l l w a y f o r G l a c i a l Lake L i l l o o e t i m m e d i a t e l y t o the n o r t h . T h i s would have i n i -t i a t e d the bedrock gorge d u r i n g a phase of f a i r l y r a p i d e r o s i o n . By the time the c o n f i n i n g i c e m e l t e d , the new channel was s u f f i c i e n t l y w e l l e s t a b l i s h e d t h a t i t was permanently m a i n t a i n e d . 65 However, the p o s s i b i l i t y remains t h a t i n both l o c a t i o n s the o l d c h a n n e l s are i n f i l l e d by l a c u s t r i n e s i l t . At the mouth of Texas Creek, where the i n f i l l i n g i s known to be s i l t , t h e r e i s some evidence of a l a t e g l a c i a l readvance from the b a s i n of Texas Creek. On the s u r f a c e of the bench here t h e r e are two low r i d g e s which cannot be e x p l a i n e d by v a l l e y a g g r a d a t i o n . One of t h e s e t r e n d s east-west a c r o s s the v a l l e y , and a s m a l l g r a v e l p i t i n i t s w e s t e r n end exposes s e v e r e l y weathered fragments of l o c a l bedrock i n t e r -mixed w i t h e r r a t i c t a l u s g r a v e l . The o t h e r r i d g e i s l o c a t e d 1\ m i l e s n o r t h of the mouth of Texas Creek and t r e n d s n o r t h -s o u t h , s t a n d i n g a l o n g the o u t e r edge of the bench. I t c o n s i s t s of w e l l rounded g r a v e l w i t h s i l t bands but the bedding i s b a d l y d i s t o r t e d , ' i n d i c a t i n g e i t h e r i c e - c o n t a c t d e p o s i t i o n or l a t e r d i s t u r b a n c e due t o o v e r - r i d i n g i c e . Both t h e s e f e a t u r e s may w e l l have, r e s u l t e d from a l a t e , l o c a l i z e d i c e readvance from the b a s i n of Texas Creek. T h i s i c e c o n f i n e d the r i v e r a g a i n s t the e a s t e r n v a l l e y w a l l and i n i t i a t e d the f o r m a t i o n of the bedrock gorge. There i s l e s s d e f i n i t e e v i d e n c e of a s i m i l a r readvance i n the area of the Jones bench. The upper c o n t a c t of the w h i t e s i l t i n the exposure a d j a c e n t to the n o r t h e r n end of the bedrock s e c t i o n of the bench i s v e r y i r r e g u l a r , F i g . 3.13. Immediately above the s i l t i s a 10' l a y e r of c o n t o r t e d s i l t y m a t e r i a l ( p o s s i b l y mudflow) t h a t i s o v e r -l a i n by a s e r i e s of mudflow g r a v e l s t h a t are an e s t i m a t e d 80' i n t h i c k n e s s . These mudflow g r a v e l s d i p towards the Jones bench silt mudflow gravel silt Id Jones bench F i g u r e end of 3.13. Jones Exposure beneath n o r t h bench; view n o r t h w a r d s . F i g u r e 3.13. Exposure beneath n o r t h e r n end of Jones bench; view northwards. 67 west, t h a t i s , away from the p r e s e n t l y l o w e s t p o i n t of the v a l l e y f l o o r , but towards the i n f e r r e d p o s i t i o n of the b u r i e d c h a n n e l . Above t h i s i s a p p r o x i m a t e l y 30' of s i l t w i t h i r r e g u l a r and u n d u l a t i n g bedding p l a n e s , i n d i c a t i n g some d i s t u r b a n c e s i n c e i t s d e p o s i t i o n . I t i s s u g g e s t e d , t e n t a t i v e l y , t h a t the r e v e r s e d i p on the mudflow g r a v e l s i n d i c a t e s t h e i r d e p o s i t i o n p r i o r to the d i v e r s i o n of the r i v e r to i t s p r e s e n t c h a n n e l . The d i v e r s i o n ( p o s s i b l y by i c e ) was r e l a t e d to the ponding t h a t i s i n d i c a t e d by the presence of the upper s i l t above the mudflow, and p o s s i b l y i t s l a t e r d i s t u r b a n c e by a c t u a l c o n t a c t w i t h the i c e . 3.2.6 R i v e r T e r r a c e s R i v e r t e r r a c e s i n the area are a l l p r o d u c t s of the phase of r i v e r d o w n c u t t i n g and t e r r a c i n g t h a t f o l l o w e d the d r a i n a g e of G l a c i a l Lake L i l l o o e t and the complete r e t r e a t of i c e from the v a l l e y f l o o r , Figs,- 3.3. The t e r r a c e s are thus d e g r a d a t i o n a l and have been c a r v e d i n the e a r l i e r Q uaternary d e p o s i t s , however, they have t h i n (up to 20') caps of d e p o s i t i o n a l c o b b l e g r a v e l . T h i s , i n t u r n , i s veneered by a 2' to 3' d e p o s i t of a e o l i a n s i l t or sand which g e n e r a l l y forms the p a r e n t m a t e r i a l f o r the l o c a l s o i l s . A l o n g i t u d i n a l p r o f i l e of the F r a s e r V a l l e y showing t e r r a c e s , benches and a l l u v i a l f a n s u r f a c e s i s g i v e n i n F i g . 3.14. On t h i s diagram, each p a r t of the s u r f a c e of a l a ndform i s i n d i c a t e d by a shaded b l o c k ; the lower l i n e bounding the b l o c k i s drawn at the e l e v a t i o n of the f r o n t FRASER V A L L E Y - L O N G I T U D I N A L P R O F I L E TO SHOW E L E V A T I O N S OF T E R R A C E S , BENCHES AND ALLUVIAL F A N S 69 edge of the s u r f a c e , t h a t i s , the l o w e s t edge of the t e r r a c e - - u s u a l l y a d j a c e n t to the r i v e r ; the upper l i n e bounding the b l o c k i s drawn at the e l e v a t i o n of the back of the t e r r a c e or bench, t h a t i s , at the j u n c t i o n of bench and h i l l s i d e b e h i n d . Thus the v e r t i c a l e x t e n t of the b l o c k i n d i c a t e s the d i f f e r e n c e i n e l e v a t i o n between the f r o n t and back of a t e r r a c e . I t i s hoped t h a t by u s i n g t h i s method o r e p r e s e n t a t i o n , i t w i l l be p o s s i b l e to c o r r e l a t e t e r r a c e s whose o u t e r edges f l u c t u a t e i n e l e v a t i o n due to v a r y i n g amounts of r i v e r e r o s i o n and h i g h t r a n s v e r s e g r a d i e n t s . With r e g a r d to the morphology of the r i v e r t e r r a c e s and o t h e r a s s o c i a t e d benches, t h i s area may be s u b d i v i d e d i n t o f i v e s e c t i o n s : (1) Along a two m i l e s t r e t c h of the v a l l e y from the mouth of the B r i d g e R i v e r southwards, the v a l l e y s i d e s are steep and very l i t t l e remains of former benches and t e r r a c e s . Narrow benches occur between e l e v a t ions cof 1 ,1 00 ' and 1,300', see F i g . 3.14. (2) In the L i l l o o e t b a s i n ( i . e . , the broad s e c t i o n of the v a l l e y around the town of L i l l o o e t , between 36 and 43 m i l e s n o r t h of L y t t o n , F i g . 3.14), the f l o o r of the v a l l e y con-s i s t s of a s e r i e s of broad r i v e r t e r r a c e s . They are non-p a i r e d and o ccur at i n t e r v a l s of about 40' between r i v e r l e v e l at 630' and the h i g h e s t t e r r a c e at 960'. T r a n s v e r s e g r a d i e n t s on these t e r r a c e s are e x t r e m e l y low, u s u a l l y l e s s than towards the c e n t e r of the v a l l e y . T h i s c h a r a c t e r -i s t i c d i s t i n g u i s h e s these t e r r a c e s from those elsewhere i n 70 the F r a s e r V a l l e y where t r a n s v e r s e g r a d i e n t s are h i g h , u s u a l l y g r e a t e r than 5°. (3) In the v i c i n i t y of the Jones bench, the bench i t s e l f i s the o n l y major t e r r a c e i n the v a l l e y . With a mean s u r -f a c e e l e v a t i o n of 1,130', i t i s of s i m i l a r e l e v a t i o n t o kame t e r r a c e s upstream and h i g h e r than a l l the a d j a c e n t r i v e r t e r r a c e s . (4) Between the s o u t h e r n margin of the Jones bench and the n o r t h e r n end of the Texas Creek bench, the v a l l e y i s a g a i n o c c u p i e d by f l i g h t s of t e r r a c e s at many e l e v a t i o n s between r i v e r l e v e l (600') and 1,200'. However, the s e t e r r a c e s are by no means as broad or c o n t i n u o u s as those i n the L i l l o o e t b a s i n . (5) In the areas both n o r t h and south of the mouth of Texas Creek the Texas Creek bench i s the o n l y major t e r r a c e i n the v a l l e y . 3.3 The F r a s e r V a l l e y between the B r i d g e R i v e r and P a v i l i o n In t h i s twenty m i l e s t r e t c h of the F r a s e r V a l l e y the benches and t e r r a c e s a t t a i n t h e i r g r e a t e s t e l e v a t i o n s above r i v e r l e v e l w i t h i n the area s t u d i e d . The w i d e s t and most prominent bench, h e r e a f t e r r e f e r r e d t o as the "main" bench, i s v i r t u a l l y c o n t i n u o u s a l o n g the e a s t e r n v a l l e y s i d e between Gibbs and P a v i l i o n C r e e k s , a d i s t a n c e of 10 m i l e s , F i g . 3.14 and 3.15. I t reaches a maximum w i d t h of over a m i l e between K e a t l e y and T i f f i n C r e e k s ; the e l e v a t i o n of FIGURE 3.15 PAVILION CREEK TO BRIDGE RIVER - LANDFORMS LEGEND low level bedrock outcrops till at surface of bench ***** till ridge kame terrace TrrrrO main bench (aggradational) ^ ? higher benches • river terraces alluvial fans minor terrace scarps earthflow PAVILION CREEK TO BRIDGE RIVER - LANDFORMS ' 72 the o u t e r edge of the bench v a r i e s between 1,400' and 1 , 300' , about 600' above r i v e r l e v e l . The bench i s a compound f e a t u r e formed by a v a r i e t y of m a t e r i a l s and d e p o s i t i o n a l p r o c e s s e s . In p l a c e s i t c o n s i s t s of two or t h r e e t e r r a c e s s e p a r a t e d by s h o r t s c a r p s l o p e s up to 40' i n h e i g h t , but commonly these have been almost o b l i t e r a t e d by s l o p e wash and s o i l c r e ep. A l s o , i n many p l a c e s the bench i s capped by a l l u v i a l f a n s stemming from the mouths of t r i b u t a r y c r e e k s ( e . g . , K e a t l e y and S a l l u s ) , which g i v e the bench a marked t r a n s v e r s e g r a d i e n t of up to 8°. The main bench i s c o n t i n u o u s a l o n g the western v a l l e y s i d e f o r a d i s t a n c e of about 3 m i l e s near S l o k and Lee Creeks and shows s i m i l a r f e a t u r e s t o those d e s c r i b e d above. Above the main bench are s e v e r a l s m a l l , i s o l a t e d t e r r a c e s l o c a t e d near the mouths of the major c r e e k s , see F i g . 3.14. These r e a c h a maximum e l e v a t i o n of almost 1,900' at the mouth of - P a v i l i o n Creek. Below the main bench t h e r e are s e v e r a l e r o s i o n a l r i v e r t e r r a c e s which have been ca r v e d out of the main bench by the a c t i o n of both the F r a s e r R i v e r and t r i b u t a r y streams. These r i v e r t e r r a c e s are most f r e q u e n t w i t h i n the embayments formed by e r o s i o n of the t r i b u t a r y streams. They are u s u a l l y narrow and occur at many d i f f e r e n t e l e v a t i o n s between t h a t of the main bench and the p r e s e n t r i v e r l e v e l . D i s s e c t i o n by the F r a s e r has formed the s t e e p - s i d e d i n n e r gorge of the v a l l e y . C o r r e s p o n d i n g d o w n c u t t i n g by t r i b u t a r y streams has formed s m a l l e r gorges and d i s s e c t e d 73 the benches and t e r r a c e s . The bedrock f l o o r of the v a l l e y i s i r r e g u l a r where i t i s t r a v e r s e d by the r i v e r at p r e s e n t . C o n s e q u e n t l y , the r i v e r has cut i n t o the bedrock to v a r y i n g d e p t h s - - i n p l a c e s bedrock b l u f f s r i s e up t o about 200' above r i v e r l e v e l - - b u t elsewhere the bedrock f l o o r i s not exposed. Three m i l e s t o the south of P a v i l i o n Creek, o p p o s i t e the mouth of Lee Cree the bedrock forms p a r t of the main bench, t h a t i s t o say, the main bench i s a r o c k - c u t bench h e r e , F i g . 3.15. The s u r f a c e of t h i s p a r t of the bench i s hummocky and i r r e g u -l a r w i t h bedrock o u t c r o p s r e s e m b l i n g t o r s . A d j a c e n t to the e a s t , an i s o l a t e d bedrock h i l l r i s e s to almost 2,000' a . s . l i t i s d r u m l i n o i d w i t h r e s p e c t t o i c e movement from n o r t h t o s o u t h . Another n o t a b l e bedrock zone o c c u r s at the mouth of Slo k Creek. The gorge t h a t t h i s creek has cut through the main bench i s almost e n t i r e l y i n bedrock, w i t h the h i g h e s t p a r t s of the bedrock not f a r below the bench s u r f a c e . The p a t t e r n of bedrock o u t c r o p s a l o n g the f l o o r of the F r a s e r V a l l e y here i n d i c a t e s a r a t h e r i r r e g u l a r shape f o r the bedrock v a l l e y , w i t h spurs e x t e n d i n g from the h i l l -s i d e s t o the c e n t r a l p a r t of the v a l l e y . T h i s p a t t e r n i s more t y p i c a l of a v a l l e y formed by f l u v i a l than by g l a c i a l p r o c e s s e s , and would t h e r e f o r e i n d i c a t e t h a t m o d i f i c a t i o n by i c e e r o s i o n was s l i g h t . . Between the mouths of S l o k and Lee C r e e k s , the F r a s e r R i v e r i s c u t t i n g i n t o r o c k s of the Spences B r i d g e group, F i g . 3.2, and these are a l s o exposed h i g h e r on the 74 e a s t e r n v a l l e y s i d e , s l i g h t l y above the l e v e l of the main bench. These v o l c a n i c r o c k s are s t r o n g l y decomposed and/or weathered and c o l o u r e d r e d , p u r p l e , and green (and e a s i l y r e c o g n i z a b l e , as a r e s u l t ) . A long the v a l l e y f l o o r , the decomposed v o l c a n i c s have been b u r i e d by the s u r f i c i a l g r a v e l s — p e r h a p s i n d i c a t i n g t h a t the d e c o m p o s i t i o n or weather-i n g took p l a c e d u r i n g s u b - a e r i a l c o n d i t i o n s p r i o r t o a g g r a d a t i o n . A l s o , they have not been eroded by the P l e i s t o c e n e i c e , a l t h o u g h c o m p a r a t i v e l y s o f t , a g a i n i n d i -c a t i n g t h a t i c e a b r a s i o n on the v a l l e y f l o o r may have been very weak. 3.3.1 S t r a t i g r a p h y of G r a v e l s beneath the Main Bench A t h i c k s e r i e s of u n c o n s o l i d a t e d beds, c o n s i s t i n g c h i e f l y o f g r a v e l w i t h some sand and s i l t , i s l o c a t e d beneath the main bench. They are w e l l exposed at i n t e r v a l s a l o n g the r i v e r banks and a l s o i n the gorges where t r i b u t a r y streams c r o s s the main bench. These g r a v e l s i n c l u d e the o l d e s t of the major Quater-nary s e d i m e n t a t i o n u n i t s at the base, and are o v e r l a i n ( i n p l a c e s ) by the most r e c e n t t i l l sheet and younger r i v e r g r a v e l s . The s t r a t i g r a p h y of the g r a v e l s was examined a l o n g the edge of the main bench between P a v i l i o n and Gibbs Creeks. Here, the ag g r , a d a t i o n a l sequence v a r i e s i n t h i c k n e s s between 0' and 700', dependent upon the e l e v a t i o n of the u n d e r l y i n g bedrock. The s t r a t i g r a p h y i s shown i n the accompanying diagram ( F i g . 3.16) and g e n e r a l i z e d i n the "1 FIGURE 3.16 FRASER VALLEY NEAR PAVILION - STRATIGRAPHY BENEATH MAIN BENCH Sallus Cr. o°0 o o o * o a oooo o oeoooo OOO^OOC-OO n o o o o o o o 1,000 0 OoOoQ 0 000-00 O ooo ooo o O O O O O o OOOOooo ooooooo Q Q Q O O O O oooooo o OoooOo miles LEGEND 0000*0 Pebble till bedded fluvial gravel cross - bedded gravel 9o° 0 - boulder gravel - sand - silt - bedrock 76 f o l l o w i n g t a b l e : -Top of s e c t i o n Maximum t h i c k n e s s ( f e e t ) U n i t 1. T i l l 150 and/or L a c u s t r i n e sand and d e l t a i c g r a v e l 100 and/or T e r r a c e g r a v e l ( e r o s i o n a l ) . 30 U n i t 2. • A g g r a d a t i o n a l r i v e r g r a v e l w i t h sand and s i l t ; i n c l u d e s t h i n beds of cobble and b o u l d e r g r a v e l 200 U n i t 3. Coarse g r a v e l bed (not every-where p r e s e n t ) 80 U n i t 4. Same as U n i t 2 150 U n i t 5. Bedrock, o v e r l a i n by l a r g e a n g u l a r bedrock b o u l d e r s or weathered zone 200 U n i t s 2 and 4 - - A g g r a d a t i o n a l R i v e r G r a v e l s These u s u a l l y c o n s i s t of mo d e r a t e l y to w e l l rounded pebbles of many d i f f e r e n t l i t h o l o g i e s . Most of the bed-roc k types of t h i s p a r t of the F r a s e r V a l l e y are found i n the g r a v e l s , and a l s o many e r r a t i c s . I n d i v i d u a l p a r t i c l e s v a r y i n s i z e from g r a n u l e t o b o u l d e r , but pebble s i z e i s u s u a l l y dominant. The g r a v e l s are m o d e r a t e l y t o w e l l s o r t e d and w e l l s t r a t i f i e d i n t h i c k beds t h a t are g e n e r a l l y h o r i -z o n t a l . Some i s o l a t e d beds of f o r e s e t or cross-bedded g r a v e l and sand do occur throughout the sequence, but they are g e n e r a l l y t h i n and not c o n t i n u o u s over any g r e a t d i s t a n c e or a l i g n e d a l o n g any r e c o g n i z a b l e l e v e l , and so are i n c l u d e d as f l u v i a l g r a v e l s . B o u l d e r beds occur at v e r t i c a l i n t e r v a l s v a r y i n g from 100' t o 200' w i t h i n the g r a v e l sequence. I n d i v i d u a l beds 77 are u s u a l l y no t h i c k e r than the diameter of one or two b o u l d e r s . The b o u l d e r s are rounded or subrounded and of many l i t h o l o g i e s , a l t h o u g h g r a n i t e tends t o be very prominent, The b o u l d e r beds are ve r y s i m i l a r i n c o m p o s i t i o n t o the c o a r s e r g r a v e l bars seen a l o n g the F r a s e r at low water at the p r e s e n t day. They may be e a s i l y t r a c e d a l o n g the v a l l e y s i d e s , s i n c e the l a r g e b o u l d e r s are an obvious f e a t u r e of the v a l l e y s l o p e s even when the t r u e o u t c r o p s are obscured by l o o s e s u r f i c i a l g r a v e l . They are c o n t i n u o u s f o r d i s t a n c e s up t o h a l f a m i l e . P o s s i b l y they r e p r e s e n t the f l o o r of a major c h a n n e l of the a g g r a d i n g F r a s e r which would s h i f t from time t o t i m e , or they may r e p r e s e n t d e p o s i t i o n by major f l o o d s . S i l t and sand u s u a l l y occur as l e n s e s t h a t can be t r a c e d f o r up to 200 or 300 ya r d s a l o n g the r i v e r banks. They may extend f a r t h e r than t h i s , but these beds do not u s u a l l y form prominent o u t c r o p s and are m o s t l y obscured by g r a v e l s from above. L o c a l l y they r e a c h maximum t h i c k n e s s e s of 20 or 30,' w i t h medium to coar s e sand beds b e i n g the;.most • epmiiro'h". " .• The h i g h e s t p r o p o r t i o n of sand and s i l t beds i s founds i n exposures between 5 to 5^ m i l e s south of P a v i l i o n Creek, see F i g . 3.16. These f i n e r m a t e r i a l s are b e l i e v e d t o r e p r e s e n t a normal p a r t of the f l u v i a l a g g r a d a t i o n a l sequence, and to have been d e p o s i t e d i n channels at times of low w a t e r , back-w a t e r s , and perhaps temporary l a k e s t h a t r e s u l t e d from f l o o d s or b l o c k i n g of the v a l l e y by l a n d s l i d e s , e a r t h f l o w or mudflow, 78 A l t h o u g h most v a r i a t i o n s i n the a g g r a d a t i o n a l sequence appear to be l o c a l , t h e r e does seem to be a s i g n i f i c a n t change i n the s t r a t i g r a p h y of these g r a v e l s at m i l e 54.3 (see F i g . 3.16),',to the northwest of Glen F r a s e r s t a t i o n . The u n u s u a l sequence here comprises u n i t 2 and c o n s i s t s of a l t e r n a t i n g g r a v e l and s i l t . The beds are h o r i z o n t a l and produce a c o n s p i c u o u s s t r i p i n g a l o n g the r i v e r banks w i t h cream c o l o u r e d s i l t and grey or brown g r a v e l s a l t e r n a t i n g . The s i l t bands d i s a p p e a r to the south u n t i l at S a l l u s Creek u n i t 2 c o n s i s t s almost e n t i r e l y of g r a v e l which i s the u s u a l s i t u a t i o n . But whether the s i l t bands change g r a d u a l l y or a b r u p t l y t o the g r a v e l i s not known s i n c e good exposures are l a c k i n g i n t h a t a r e a . P o s s i b l y the u n u s u a l s i l t and g r a v e l sequence i s a r e f l e c t i o n of the p r o x i m i t y of the a l l u v i a l f a n s at the mouths of K e a t l e y and S a l l u s Creeks. Then the s i l t beds c o u l d e i t h e r be of l a c u s t r i n e o r i g i n , w i t h the l a k e s r e s u l t i n g from a s e r i e s of temporary b l o c k a g e s of the v a l l e y by the l a t e r a l a g g r a d a t i o n of the f a n s , or a l t e r n a t e l y , the " s i l t " beds c o u l d c o n s i s t of the s i l t y mudflow m a t e r i a l t h a t i s the c h a r a c t e r i s t i c d e p o s i t on the fans here. (Most of the s i l t beds are i d e n t i f i e d by the e f f e c t of t h e i r c o l o u r on the ground, r a t h e r than from o b s e r v i n g the s i l t i n good e x p o s u r e s , so i t i s not known whether these beds c o n s i s t of pure s i l t or of s i l t y mudflow g r a v e l . ) 79 U n i t 3--An E x c e p t i o n a l l y Coarse G r a v e l Bed A-n e x c e p t i o n a l l y c o a r s e and t h i c k g r a v e l bed oc c u r s w i t h i n the a g g r a d a t i o n a l sequence at an e l e v a t i o n of between 1,100' and 1,200', t h a t i s , about t w o - t h i r d s of the way up between r i v e r l e v e l and the s u r f a c e of the main bench. I t s e l e v a t i o n d e creases s l i g h t l y downstream, see F i g . 3.16. I t i s composed p r e d o m i n a n t l y of sub-rounded c o b b l e s -and b o u l d e r s up to 5' diameter and of mixed l i t h o l o g i e s . In d e t a i l , the i n t e r n a l s t r u c t u r e and c o m p o s i t i o n of the bed vary from p l a c e t o p l a c e , a l t h o u g h from a d i s t a n c e i t always forms a prominent f e a t u r e w i t h the l a r g e p a r t i c l e s and almost v e r t i c a l s l o p e , see F i g . 3.17. In p l a c e s beds of sand and f i n e g r a v e l are i n c l u d e d w i t h i n t h i s c o a r s e bed. The l a r g e r b o u l d e r s occupy the margins of the f i n e r l e n s e s , f o r m i n g i r r e g u l a r beds t h a t may be o n l y one or two b o u l d e r s i n t h i c k n e s s , s i m i l a r t o the b o u l d e r beds w i t h i n the a g g r a d a t i o n a l g r a v e l s , F i g . 3.18. E l s e w h e r e , the bed c o n s i s t s e n t i r e l y of b o u l d e r s t h a t are c l o s e l y packed t o g e t h e r w i t h no v i s i b l e bedding. I t reaches i t s maximum t h i c k n e s s of 80' when i t has the s e c h a r a c t e r i s t i c s . To the south of T i f f i n Creek the coar s e g r a v e l con-s i s t s of two d i s t i n c t u n i t s : an upper 30' bed of ve r y c o a r s e , b o u l d e r and co b b l e g r a v e l , c l e a n , l i g h t grey i n c o l o u r , and p o o r l y bedded; the lower bed i s of s i m i l a r t h i c k n e s s but w i t h a s l i g h t l y h i g h e r p r o p o r t i o n of a n g u l a r m a t e r i a l and d i s t i n g u i s h e d c h i e f l y by i t s d i r t y brown F i g u r e 3.17. The F r a s e r V a l l e y between P a v i l i o n and Lee C r e e k s : view upstream showing the main bench w i t h g r a v e l s exposed i n the r i v e r banks beneath; note the c o a r s e g r a v e l bed s t a n d i n g w i t h a v e r t i c a l f a c e ( l o c a t e d at a p p r o x i m a t e l y t w o - t h i r d s of the h e i g h t between r i v e r and main bench) and the bedrock exposures near r i v e r l e v e l . F i g u r e 3.18. F r a s e r Valley-near P a v i l i o n : coarse g r a v e l bed - U n i t 3 - b o u l d e r beds and i r r e g u l a r l e n s e s of f i n e r m a t e r i a l . F i g u r e 3.20. F r a s e r V a l l e y near P a v i l i o n : cross-bedded g r a v e l near s u r f a c e of main bench o v e r l y i n g l a c u s t r i n e sand w i t h i c e - r a f t e d b o u l d e r s . 82 c o l o u r due to a h i g h c o n t e n t of d i r t y s i l t m a t r i x . T h i s c o a r s e g r a v e l bed i s not c o n t i n u o u s throughout the P a v i l i o n - G i b b s s e c t i o n of the v a l l e y , but appears to fade towards the h i g h e r bedrock b l u f f s t h a t form the lower p a r t s of the r i v e r banks i n many p l a c e s . The c o a r s e g r a v e l i s absent above the b l u f f s - , ./where" i t i s l o c -a l l y r e p l a c e d . b y s i l t and sand beds a l o n g a p p r o x i m a t e l y the same h o r i z o n . Where they are h i g h e s t , the bedrock b l u f f s r i s e to s l i g h t l y below the l e v e l of the c o a r s e r g r a v e l and so i t i s not c l e a r how they e x e r t e d any c o n t r o l upon the d e p o s i t i o n of t h i s bed. P o s s i b l y t h e i r upward c o n t i n u a t i o n ( b u r i e d by g r a v e l s at the p r e s e n t time) formed p a r t i a l c o n s t r i c t i o n s t o the v a l l e y at the time of d e p o s i t i o n of the c o a r s e g r a v e l and thus i n f l u e n c e d i t s c o m p o s i t i o n , F i g . 3.16. I t i s suggested t h a t t h i s c o a r s e g r a v e l bed o r i g i n a t e d d u r i n g a phase of r i v e r d e g r a d a t i o n t h a t i n t e r p o s e d between the two phases of a g g r a d a t i o n as i n d i c a t e d by u n i t s 2 and 4-of the g r a v e l sequence. Most of the s t r a t i g r a p h i c c h a r a c -t e r i s t i c s of t h i s bed may be observed i n f o r m a t i o n a l o n g the modern ch a n n e l of the p r e s e n t l y d e g r a d i n g F r a s e r R i v e r . Coarse m a t e r i a l accumulates as a l a g d e p o s i t from the e r o s i o n of u n c o n s o l i d a t e d m a t e r i a l a l o n g the r i v e r banks and i s a l s o d e r i v e d from l o c a l bedrock o u t c r o p s . J u x t a p o s i t i o n of l a r g e b o u l d e r s and sand i s observed at low water s t a g e s . The t w o - f o l d d i v i s i o n of the bed south of T i f f i n Creek c o u l d p o s s i b l y r e p r e s e n t the i n c o r p o r a t i o n of mudflow or slumped 83 m a t e r i a l i n t o the r i v e r c h a n n e l . D e p o s i t i o n of such a bed by i c e i s u n l i k e l y , s i n c e no mechanism e x i s t s f o r the s e l e c t i v e a c c u m u l a t i o n of c o a r s e m a t e r i a l . A l s o , the r e l a t i v e l y u n i f o r m t h i c k n e s s and e l e -v a t i o n are not c h a r a c t e r i s t i c s commonly a s s o c i a t e d w i t h a t i l l s h eet. 3.3.2 U n i t 1--The Pebble T i l l C l o s e beneath the s u r f a c e of much of the main bench t h e r e i s a l a y e r of t i l l . T h i s i s r e f e r r e d t o as the "pebble t i l l " s i n c e i t c o n s i s t s c h i e f l y of rounded pebbles which are thought to have been d e r i v e d by r e w o r k i n g from f l u v i a l g r a v e l s . The t i l l was found i n many l o c a t i o n s ; i t c h a r a c t e r i -s t i c a l l y caps the main bench, o v e r l y i n g the f l u v i a l a g g r a d a t -i o n a l g r a v e l s between P a v i l i o n and S a l l u s Creeks. I t i s a l s o found a t the mouth of Gibbs Creek i n the r a i l r o a d c u t t i n g at the back of the main bench (where i t i s b u r i e d by e a r t h f l o w m a t e r i a l ) and beneath the main bench and a narrow o u t e r bench where i t i s capped by e a r t h f l o w and mudflow m a t e r i a l . The pebble t i l l i s a l s o exposed i n r a i l r o a d c u t t i n g s downstream from the mouth of F o u n t a i n Creek where i t reaches i t s g r e a t e s t t h i c k n e s s . From here i t grades downstream i n t o a kaine complex l o c a t e d between one and two m i l e s upstream from the mouth of the B r i d g e R i v e r . Compos i t i o n The pebble t i l l o c c u r s as a massive bed of rounded p e b b l e s w i t h a s i l t y m a t r i x . The pebbles are r e l a t i v e l y 84 s m a l l , most of them b e i n g l e s s than 3" i n d i a m e t e r , but l o c a l l y , c o b b l e and b o u l d e r s i z e d m a t e r i a l i s i m p o r t a n t . The peb b l e s are of many l i t h o l o g i e s - - m o s t of the r e g i o n a l rock types are r e p r e s e n t e d . The t i l l i s a l s o c h a r a c t e r i z e d by two k i n d s of i n c l u s i o n s . B o u l d e r s of s i l t and i r r e g u l a r s i l t l e n s e s ( c o n s o l i d a t e d s i l t beds t h a t have been eroded en mass, d i s t o r t e d , and. then r e d e p o s i t e d ) are common i n the pebble t i l l t h r oughout a l l t h e s e exposures where i t was found. A more s t r i k i n g form of i n c l u s i o n - ' i.s lumps of b r i g h t r e d m a t e r i a l which' c o n s i s t of rounded grey f l u v i a l p e b b l e s s et i n a r e d c l a y e y m a t r i x . The pebbles are s i m i l a r t o those i n the main body of the t i l l . These r e d i n c l u s i o n s are a ve r y obvious f e a t u r e of the t i l l , where they o c c u r , and so he l p i n i t s i d e n t i f i c a t i o n , F i g . 3.19. These r e d i n c l u s i o n s show g r e a t v a r i a t i o n both i n s i z e and i n fr e q u e n c y of o c c u r r e n c e . They occur as rounded or a n g u l a r masses t h a t range i n s i z e from a few i n c h e s to over 30' a c r o s s . They are not d i s t r i b u t e d e v e n l y throughout the pebble t i l l , but tend t o occur i n c l u s t e r s , each c l u s t e r perhaps produced by the d i s i n t e g r a t i o n of a larger- i n c l u s i o n . They are most common i n the area between P a v i l i o n and S a l l u s C r e e k s , and decrease i n f r e q u e n c y and s i z e southwards. The most s o u t h e r l y i n c l u s i o n t h a t was found was l o c a t e d about h a l f a m i l e downstream from F o u n t a i n s t a t i o n . I t i s b e l i e v e d t h a t these, i n c l u s i o n s are d e r i v e d from the Coldwater Beds of Miocene age or e a r l i e r as d e s c r i b e d F i g u r e Pebble 3.19. F r a s e r T i l l w i t h r e d V a l l e y near P a v i l i o n : the i n c l u s i o n s of Coldwater Beds. 86 by D u f f e l l and McTaggart (1952) and T r e t t i n (1960). These i n c l u d e some beds of conglomerate which, i n p l a c e s , are r e p o r t e d to be " s c a r c e l y c o n s o l i d a t e d as the pebbles c o u l d be broken out by hand" ( D u f f e l l and McTaggart, 1952 , p. 64-). The roundstones i n the conglomerate are p e b b l e - s i z e d and c o n s i s t of c h e r t , g r e e n s t o n e , l i m e s t o n e , s h a l e and a s m a l l p e r c e n t a g e of l a v a and b a t h o l i t h i c r o c k s . These a p p r o x i m a t e l y c o r r e s p o n d to the l i t h o l o g i e s t h a t were observed w i t h i n the i n c l u s i o n s by the w r i t e r . These T e r t i a r y conglomerates out-crop on the s l o p e s of the F r a s e r V a l l e y t o the south of P a v i l i o n Creek at e l e v a t i o n s between 3,000' and 3,500' where they form a b e l t about a m i l e l o n g and one e i g h t h of a m i l e w i d e . I f t h i s was the o r i g i n of the r e d i n c l u s i o n s , then t h e i r d i s t r i b u t i o n may be expected to g i v e some i n d i c a t i o n of the d i r e c t i o n of i c e movement, which must t h e r e f o r e have been from n o r t h t o s o u t h , f o l l o w i n g the t r e n d of the F r a s e r V a l l e y , and w i t h a s m a l l w e s t e r l y component to b r i n g the i n c l u s i o n s from the e a s t e r n v a l l e y s i d e towards the c e n t e r . T h i s movement i s d i f f e r e n t from t h a t which has been r e c o n -s t r u c t e d from s t r i a e i n the g e n e r a l a r e a , which show the main i c e movement ( f o r the C o r d i l l e r a n g l a c i e r ) to have been so u t h -eastwards. Any v a l l e y g l a c i e r here d u r i n g a l a t e g l a c i a l stage might be expected to have f l o w e d from south t o n o r t h , upstream, but away from the Coast Range mountains which i s the most l i k e l y l o c a l a c c u m u l a t i o n a r e a . 87 The movement i m p l i e d from the l o c a t i o n s of the i n c l u s i o n s and t h e i r source would suggest a v a l l e y g l a c i e r moving from n o r t h t o s o u t h , t h e r e b y c r e a t i n g a problem as to the source of the i c e . In view of these c o n f l i c t i n g s i t u a t i o n s , i t i s suggested t h a t the t r a n s p o r t a t i o n of the i n c l u s i o n s of Coldwater Beds may have been due to a g e n c i e s o t h e r than the d i r e c t f l o w of the i c e . For example, the m a t e r i a l may have been o r i g i n a l l y c a r r i e d out onto the i c e by an e a r t h f l o w (as took p l a c e from the Coldwater Beds on the e a s t e r n s i d e of P a v i l i o n ) , and then d i s t r i b u t e d by m e l t -water streams f l o w i n g on the i c e s u r f a c e from n o r t h t o south d u r i n g the m e l t i n g phase. Topography A l t h o u g h the upper s u r f a c e of the pebble t i l l i n p l a c e s l i e s v e r y c l o s e t o the s u r f a c e of the main bench, t h e r e are v e r y few t o p o g r a p h i c f e a t u r e s a t t r i b u t a b l e t o g l a c i a l d e p o s i t i o n . Two m i l e s to the n o r t h of S a l l u s Creek the t i l l i s exposed on the s u r f a c e of the main bench over a s m a l l t r i a n g u l a r a r e a , see F i g . 3.15. T h i s i s an area of g e n t l y r o l l i n g t opography, but to a ground o b s e r v e r i t i s not n o t i c e a b l y d i f f e r e n t t o the s u r f a c e of the bench else w h e r e . Along the o u t e r edge of t h i s bench i s a narrow and steep s i d e d r i d g e composed of t i l l and up to 40' i n h e i g h t . I t s o r i g i n i s u n c e r t a i n , but i t i s p o s s i b l y some k i n d of a m o r a i n i c f e a t u r e . 88 On the o u t e r edge of the bench t h a t i s im m e d i a t e l y t o the west of F o u n t a i n Creek i s a low, hummocky m o r a i n i c r i d g e w i t h l a r g e a n g u l a r b o u l d e r s of l o c a l bedrock on the s u r f a c e . F i g . 3.15. The pebble t i l l i s exposed i n a r a i l r o a d c u t t i n g beneath the edge of the t e r r a c e . T h i s i s the o n l y l o c a t i o n where a n y t h i n g t h a t might be d e s c r i b e d as " t y p i c a l m o r a i n i c topography" was found. E l s e w h e r e , i t appears t h a t the t i l l was e i t h e r p l a n e d o f f by r i v e r a c t i o n and th e r e b y f l a t t e n e d , or b u r i e d by a t h i n l a y e r of f l u v i a l d e p o s i t s . The upper s u r f a c e of the pebble t i l l i s commonly t r a n s i t i o n a l t o the p o o r l y bedded f l u v i a l g r a v e l s which l i e i m m e d i a t e l y below the s u r f a c e of the main bench and c o n t r o l i t s c o n f i g u r a t i o n . In o t h e r i n s t a n c e s , the t i l l i s o v e r l a i n by a t h i n t e r r a c e c a p p i n g of cob b l e g r a v e l , s i m i l a r t o t h a t which c h a r a c t e r i s t i c a l l y caps the e r o s i o n a l r i v e r t e r r a c e s . B a s a l C o n t a c t of the Pebble T i l l The b a s a l c o n t a c t of the pebble t i l l has been observed to be ' - g r a d a t i o n a l . . ' i n some l o c a t i o n s - b u t > e r o s i o n a l i n ot h e r s. In many p l a c e s a l o n g the edge of the main bench, p a r t i c u l a r l y between P a v i l i o n and S a l l u s C r e e k s , d e l t a i c ' g r a v e l was found to be c l o s e l y a s s o c i a t e d w i t h the t i l l . I t e i t h e r l i e s i m m e d i a t e l y beneath the t i l l or o c c u p i e s the same s t r a t i g r a p h i c p o s i t i o n a l o n g the upper p a r t of the main bench. The d e l t a i c g r a v e l s u s u a l l y c o n s i s t of f o r e s e t beds, 89 up t o 50' i n t h i c k n e s s which show no predominant d i p d i r e c t i o n . I n f r e q u e n t l y the b a s a l p a r t of the t i l l was found i n t e r b e d d e d w i t h the uppermost beds of d e l t a i c g r a v e l , i n d i c a t i n g t h a t v e r y l i t t l e time e l a p s e d d u r i n g the r e p l a c e -ment of d e l t a i c by g l a c i a l c o n d i t i o n s . Beneath the f o r e s e t beds t h e r e i s u s u a l l y a bed of medium-textured sand, l a m i n a t e d and h o r i z o n t a l l y bedded, and w i t h a maximum t h i c k n e s s of 40'. The sand f r e q u e n t l y c o n t a i n s i s o l a t e d p e b b l e s and c o b b l e s , p r o b a b l y t r a n s p o r t e d to t h i s p o s i t i o n by i c e - r a f t i n g , F i g . 3.20. T h i s e n t i r e sequence i s i n t e r p r e t e d as r e s u l t i n g from l a c u s t r i n e c o n d i t i o n s accompanying the ad v a n c i n g g l a c i e r . The sand, o v e r l a i n by d e l t a i c g r a v e l , i n t u r n o v e r l a i n by t i l l , i s the sequence of d e p o s i t i o n t o be expected under these c i r c u m s t a n c e s . Where the b a s a l c o n t a c t i s e r o s i o n a l , the t i l l t r u n c a t e s the u n d e r l y i n g g r a v e l s . A l t h o u g h the amount of e r o s i o n a t t r i b u t a b l e t o the pebble t i l l g l a c i a t i o n i s not known, t h e r e are s e v e r a l i n d i c a t i o n s t h a t i t was p r o b a b l y o n l y s l i g h t . F i r s t l y , both e r o s i o n a l and conformable b a s a l c o n t a c t s of the t i l l o ccur w i t h i n the same g e n e r a l area and at the same g e n e r a l e l e v a t i o n s . S e c o n d l y , the t i l l i s com-posed of f a i r l y u n i f o r m , p e b b l e - s i z e d p a r t i c l e s d e r i v e d from the uppermost f l u v i a l l a y e r s over which the i c e moved; i f e r o s i o n by the i c e had been v e r y s e v e r e , a f a r g r e a t e r amount of m i x i n g of m a t e r i a l from d i f f e r e n t h o r i z o n s might have been e x p e c t e d , and t h e r e f o r e a much g r e a t e r v a r i a t i o n i n 90 the n a t u r e of the pebble t i l l . T h i r d l y , the pebble t i l l i s a c h a r a c t e r i s t i c a l l y t h i n c a p p i n g to the main bench, b e i n g u s u a l l y l e s s than 50' i n t h i c k n e s s ; s m a l l volume of t i l l prob-a b l y i n d i c a t e s s m a l l amount of e r o s i o n by the i c e t h a t dumped i t . The most severe e r o s i o n by the pebble t i l l i c e seems to have taken p l a c e i n the c o n s t r i c t e d p o r t i o n of the v a l l e y around the "S" bend downstream from F o u n t a i n Creek. Near t h i s creek the u n d e r l y i n g beds of g r a v e l , sand and s i l t have been d i s t u r b e d and c o n t o r t e d by the p r e s s u r e of the o v e r - r i d i n g i c e down to depths of 40' below the t i l l - f l u v i a l c o n t a c t . At one l o c a t i o n h e r e , s m a l l f r a c t u r e s developed i n sand beds by t h i s p r e s s u r e i n d i c a t e an u p - v a l l e y d i r e c t i o n of i c e f l o w . A l s o i n t h i s g e n e r a l a r e a , s e v e r a l exposures were observed where the c o n t a c t between t i l l and e a r l i e r f l u v i a l m a t e r i a l was v e r t i c a l , p r o b a b l y i n d i c a t i n g t h a t i c e e r o s i o n s e v e r e l y a f f e c t e d the. e a r l i e r d e p o s i t s . At the mouth of F o u n t a i n Creek the pebble t i l l o v er-l i e s f l u v i a l g r a v e l s w i t h the c o n t a c t at T,150'. Between 2 and 4 m i l e s f u r t h e r downstream the t i l l extends unbroken down to bedrock at a p p r o x i m a t e l y 800'. Hence over a r e l a t i v e l y s h o r t d i s t a n c e t h e r e i s a d i f f e r e n c e i n e l e v a t i o n of 300' f o r the base of the t i l l . A l t h o u g h t h i s d i f f e r e n c e c o u l d s i m p l y r e f l e c t the form of the p r e - g l a c i a l topography, t h i s i s u n l i k e l y s i n c e f a r t h e r upstream the s u r f a c e upon which the t i l l r e s t s conformably ( i . e . , the p r e - g l a c i a l s u r f a c e ) i s v i r t u a l l y h o r i z o n t a l . Thus i t would appear t h a t at l e a s t 300' of e r o s i o n was c a r r i e d out by t h i s i c e i n the 91 n a r r o w e r , and downstream p a r t of the v a l l e y . 3.3.3 The F o u n t a i n Kame Between 2 and 4 m i l e s downstream from F o u n t a i n Creek the pebble t i l l c o mprises the e n t i r e s e c t i o n of u n c o n s o l i -dated m a t e r i a l between bedrock and the s u r f a c e of the l a r g e s t t e r r a c e , as mentioned above. Between here and the mouth of the B r i d g e R i v e r s t r a t i f i e d d e p o s i t s of s i l t , sand and g r a v e l appear w i t h i n the t i l l . These are u s u a l l y i n the form of i r r e g u l a r l e n s e s and are i n t e r p r e t e d as w a t e r l a i d m a t e r i a l t h a t was d e p o s i t e d i n c o n t a c t w i t h the i c e , and d i s t u r b e d and c o n t o r t e d e i t h e r due to i c e movement or due to c o l l a p s e d u r i n g i c e m e l t i n g . The whole complex of these d e p o s i t s i s c l a s s i f i e d as a kame, b u i l t d u r i n g the f i n a l s t ages of g l a c i a t i o n i n t h i s s e c t i o n of the v a l l e y . Beneath the main r i v e r t e r r a c e at 1,200' to 1,300' e l e v a t i o n to the west of the mouth of F o u n t a i n Creek, the pebble t i l l - k a r a e complex has been b u r i e d by l a t e r f l u v i a l g r a v e l s , but between F o u n t a i n s t a t i o n and the B r i d g e R i v e r the o r i g i n a l c o n s t r u c t i o n a l s u r f a c e of the kame remains to the p r e s e n t time ( p a r t i a l l y eroded by the F r a s e r R i v e r ) , F i g . 3.15. I t forms a bench w i t h a markedly u n d u l a t i n g s u r f a c e between 1,100' and 1,400' and i n c l u d i n g some c l o s e d d e p r e s s i o n s . Remnants of a s i m i l a r f e a t u r e are t o be found on the o p p o s i t e v a l l e y s i d e , and a l s o as a v e r y narrow bench al o n g both s i d e s of the v a l l e y between the mouth of the B r i d g e R i v e r and the r a i l r o a d b r i d g e n o r t h of L i l l o o e t . 92 Thus i t would appear t h a t the narrow s e c t i o n of the F r a s e r V a l l e y between L i l l o o e t and F o u n t a i n s t a t i o n was the s i t e of s t a g n a t i n g i c e d u r i n g the l a s t d e g l a c i a t i o n f o l l o w i n g the d e p o s i t i o n of the pebble t i l l . 3.3.4 Other G l a c i a l D e p o s i t s A second type of g l a c i a l t i l l was found as p a r t of a complex s e r i e s of d e p o s i t s a t the mouth of S a l l u s Creek. The t i l l o c c u r s beneath the apex of the S a l l u s a l l u v i a l f a n at an e l e v a t i o n of a p p r o x i m a t e l y 1,600'. I t i s o v e r l a i n by a t h i c k s e r i e s of f a n mudflow g r a v e l s and exposed o n l y i n the r a i l r o a d c u t t i n g . T h i s t i l l d i f f e r s c o n s i d e r a b l y from the pebble t i l l s i n c e i t i s composed d o m i n a n t l y of a n g u l a r r o c k fragments up to 2.5' d i a m e t e r s e t i n a s i l t - s a n d m a t r i x . Some i s o l a t e d beds of s i l t and f i n e g r a v e l occur w i t h i n the t i l l , w h i l s t f u r t h e r down-fan the t i l l grades i n t o s i l t y g r a v e l s t h a t show a rough s t r a t i f i c a t i o n . P o s s i b l y t h i s S a l l u s t i l l r e p r e s e n t s a l o c a l v a r i a t i o n on the pebble t i l l and i s contemporaneous w i t h i t . I t may have been dumped from the margin of the g l a c i e r t h a t was a b r a d i n g bedrock whereas the pebble t i l l was formed beneath the c e n t r a l p a r t of the g l a c i e r t h a t was r e w o r k i n g the e a r l i e r f l u v i a l d e p o s i t s . E v i d e n c e f o r t h i s i s t h a t the S a l l u s t i l l does appear to occupy a s i m i l a r s t r a t i g r a p h i c p o s i t i o n to the pebble t i l l : both r e s t upon t h i c k beds of f l u v i a l g r a v e l , and both are o v e r l a i d by r e c e n t mudflow and/or e a r t h f l o w . 93 3.3.5 A g g r a d a t i o n a l G r a v e l s above the Main Bench The main bench does not mark the upper l i m i t of a g g r a d a t i o n i n the v a l l e y . Where t r i b u t a r y streams e n t e r the main v a l l e y , t h e r e are commonly s e v e r a l t e r r a c e s i n the stream r e e n t r a n t t h a t stand w e l l above the e l e v a t i o n of the main bench, F i g s . 3.14, 3.15. These have been d i s s e c t e d d u r i n g the most r e c e n t phase of d o w n c u t t i n g and f l u v i a l g r a v e l s are commonly exposed a l o n g the g u l l i e s formed i n t h i s manner. The h i g h e s t exposure found i s i n the modern g u l l y of P a v i l i o n Creek where a s e r i e s of f i n e g r a v e l s , w e l l s o r t e d and h o r i z o n t a l l y bedded i s exposed at 1,880'. The most l i k e l y o r i g i n f o r these h i g h - l e v e l stream g r a v e l s and t e r r a c e s i s by m a r g i n a l d e p o s i t i o n as kames when i c e s t i l l o c c u p i e d the c e n t r a l p a r t of the v a l l e y . For these t e r r a c e s to be remnants of a g e n e r a l phase of a g g r a d a t i o n would r e q u i r e t h e i r d e p o s i t i o n to have been f o l l o w e d by removal of the upper 400' of g r a v e l , u n i f o r m l y over a broad s e c t i o n of the v a l l e y , to form the main bench. T h i s i s con-s i d e r e d u n l i k e l y , s i n c e the pebble t i l l i c e seems to have c a r r i e d out o n l y a minor amount of e r o s i o n , and i t was i m m e d i a t e l y preceded by a g g r a d a t i o n up to the e l e v a t i o n of the main bench. To the n o r t h of t h a t p a r t of the F r a s e r V a l l e y t h a t was s t u d i e d i n d e t a i l , a sequence of a g g r a d a t i o n a l m a t e r i a l i s exposed i n the r i v e r banks t h a t appears t o d i f f e r c o n s i d e r -a b l y from the a g g r a d a t i o n a l g r a v e l s below the main bench (and d e s c r i b e d above). T h i s m a t e r i a l appears on the e a s t e r n v a l l e y 94 s i d e about one m i l e to the n o r t h of P a v i l i o n Creek and on the western v a l l e y s i d e t o the n o r t h of S l o k Creek. ( P o s s i b l y the bedrock r i d g e through which S l o k Creek has cut the lower p a r t of i t s course i s connected w i t h t h i s t r a n s i t i o n . ) The m a t e r i a l here appears.;to c o n s i s t of a l t e r n a t i n g l a y e r s of g r a v e l and s i l t t h a t extend from the f l o o r of. the v a l l e y up to 1,000' above r i v e r l e v e l . The upper l i m i t i s marked by a broad s l o p i n g bench on the w e s t e r n v a l l e y s i d e , t h a t stands c o n s i d e r a b l y h i g h e r (above r i v e r l e v e l ) than the main bench, F i g . 3.21. ''''%K;'"y "' T h i s change i n the n a t u r e of the d e p o s i t s t o g e t h e r w i t h the change i n e l e v a t i o n of the b r o a d e s t bench i n the v a l l e y , would i n d i c a t e t h a t the h i s t o r y of the two s e t s of a g g r a d a t i o n a l m a t e r i a l and t h e i r r e s p e c t i v e benches d i f f e r , w i t h the t h i c k e r s i l t - g r a v e l sequence n o r t h of P a v i l i o n b e i n g t h e . o l d e r . An a l t e r n a t i v e p o s s i b i l i t y r e g a r d i n g the o r i g i n of the h i g h - l e v e l stream t e r r a c e s would be t h a t they are e r o s i o n a l remnants of t h i s o l d e r sequence. 3.3.6 The S t r u c t u r e of the P l e i s t o c e n e D e p o s i t s Between P a v i l i o n and Gibbs C r e e k s , the r e l a t i o n s h i p between the v a r i o u s s e d i m e n t a r y u n i t s i s thought to be as i n d i c a t e d i n F i g . 3.22. The main bench marks the upper l i m i t of the most r e c e n t phase of a g g r a d a t i o n ; the e a r l i e r and t h i c k e r s i l t - g r a v e l u n i t , as exposed n o r t h of P a v i l i o n , p o s s i b l y extend f u r t h e r south as e r o s i o n a l remnants a l o n g the v a l l e y s i d e s and may u n d e r l i e s e c t i o n s of the benches f u r t h e s t F i g u r e 3.21. F r a s e r V a l l e y near P a v i l i o n : view northwards to the n o r t h e r n bank of P a v i l i o n Creek showing p a r t of main bench and g r a v e l s beneath; note " s t r i p e d " v a l l e y s i d e s beyond. F i g u r e 3.21. F r a s e r V a l l e y near P a v i l i o n : view northwards n o r t h e r n bank of P a v i l i o n Creek showing p a r t of main bench g r a v e l s beneath; note " s t r i p e d " v a l l e y s i d e s beyond. to the and FIGURE 3.22 CROSS-SECTION OF FRASER VALLEY NEAR PAVILION SHOWING INFERRED RELATIONSHIP BETWEEN DEPOSITS FEET . 3 ,000 r 2,500 2 , 0 0 0 1500 1,000 5 0 0 h v bedrock vertical exaggeration X 4 approximately LEGEND O O O striped sequence - Pebble Ti l l (Unit I ) - aggradational gravels (Units 2 and 4 ) lo o o o <300i>ol coarse gravel (Unit 3 ) alluvial fan high terrace gravels 97 away from the r i v e r . The h i g h l e v e l stream t e r r a c e may be p a r t of t h i s u n i t , or belon g t o a l a t e r i c e - m a r g i n a l phase. Downstream from Gibbs Creek the pebble t i l l becomes i n c r e a s i n g l y i m p o r t a n t , ' u n t i l between F o u n t a i n s t a t i o n and the mouth of the B r i d g e R i v e r i t c o n s t i t u t e s most of the volume of u n c o n s o l i d a t e d m a t e r i a l i n the v a l l e y . 3.3.7 P o s t - G l a c i a l D e p o s i t i o n F l u v i a l d e p o s i t i o n i n p o s t - g l a c i a l time i s l i m i t e d t o s m a l l areas of t h i n f l u v i a l g r a v e l s on the main bench, and the c o b b l e - g r a v e l s t h a t cap c e r t a i n areas of the main bench and the lower e r o s i o n a l t e r r a c e s formed d u r i n g the d o w n c u t t i n g phase of the F r a s e r and i t s t r i b u t a r i e s . Mudflow d e p o s i t s were found c a p p i n g the main bench i n the v i c i n i t y of K e a t l e y and S a l l u s C r e e k s , where the bench has been m o d i f i e d i n t o the form of a l l u v i a l f a n s by t h i s p r o c e s s . Near the c e n t e r of the v a l l e y the mudflow beds f r e q u e n t l y r e s t upon t h i n beds of grey t e r r a c e g r a v e l , i n d i -c a t i n g t h a t the fans were b u i l t onto t e r r a c e s r e c e n t l y occu-p i e d by the F r a s e r . These mudflow d e p o s i t s are d i s c u s s e d i n more d e t a i l i n S e c t i o n 10.1.2. Between S a l l u s and F o u n t a i n Creeks l a r g e areas of the h i l l s i d e s above the main bench have been a f f e c t e d by e a r t h f l o w , These are most e a s i l y i d e n t i f i e d by the form of the ground s u r f a c e , which i s c h a r a c t e r i s t i c a l l y i r r e g u l a r and hummocky. The e a r t h f l o w m a t e r i a l i s w e l l exposed i n numerous road and r a i l r o a d c u t t i n g s , p a r t i c u l a r l y the r a i l r o a d c u t t i n g a t Gibbs Creek where the c o l l u v i a l m a t e r i a l unconformably 98 o v e r l i e s the pebble t i l l . I t c o n s i s t s of i r r e g u l a r fragments of the l o c a l bedrock ( c h i e f l y d a c i t e and r h y o l i t e of the Spences B r i d g e group) s e t i n a m a t r i x of s i l t y s o i l and s m a l l e r f r a g m e n t s . The local''-, fragment's v a r y i n s i z e up to 10' diameter and have been bent and c o n t o r t e d by the movement. Many of the l a r g e r p i e c e s have undergone severe w e a t h e r i n g r e s u l t i n g i n b r i g h t c o l o u r s of g r e e n , r e d and p u r p l e . The presence of ro c k fragments of s i m i l a r t y p e , but unweathered, would i n d i c a t e t h a t s t r o n g w e a t h e r i n g had ta k e n p l a c e p r i o r to the removal of the m a t e r i a l by the e a r t h f l o w . S i n c e the area of e a r t h f l o w s a p p r o x i m a t e l y c o r r e s p o n d s to the ou t c r o p of the r o c k s of the Spences B r i d g e Group, i t may be supposed t h a t the ro c k type and i t s b a d l y weathered s t a t e was an im p o r t a n t c o n t r i b u t a r y cause of the e a r t h f l o w . In p l a c e s the m a t e r i a l of the e a r t h f l o w was found to be i n t e r b e d d e d w i t h h o r i z o n t a l l a y e r s of s i l t , sand and f i n e g r a v e l . The s i l t and sand i n v o l v e d i s commonly ripple'-- . marked. T h i s p o s s i b l y i n d i c a t e s t h a t f l u v i a l a g g r a d a t i o n of the v a l l e y f l o o r was s t i l l i n p r o g r e s s when the e a r t h f l o w s o c c u r r e d , F i g . 3.23. 3.4 The F r a s e r V a l l e y between Texas Creek and L y t t o n Along t h i s s e c t i o n of the F r a s e r V a l l e y the main bench i s a c o n s t r u c t i o n a l s u r f a c e s t a n d i n g between 400' and 500' above the p r e s e n t r i v e r bed ( i n d i c a t e d by t r e n d l i n e (2) on F i g . 3.14). I t i s by no means a c o n t i n u o u s f e a t u r e , but has been s e v e r e l y eroded by the F r a s e r and d i s s e c t e d by FIGURE 3.23 SECTIONS AT 1,250' NEAR FOUNTAIN SHOWING INTERBEDDED FLUVIAL AND EARTHFLOW MATERIAL 10 ft. > A O o o o o oo O o o o o o o o <fj> ^ a0O000Oo a-- earthflow - fluvial gravel - sand - silt and .sand with ripple marks 100 t r i b u t a r y streams s i n c e i t s f o r m a t i o n . Below the main bench are narrow r i v e r t e r r a c e s t h a t were formed d u r i n g the most r e c e n t d o w n c u t t i n g of the F r a s e r . These are most numerous i n the broader p a r t of the v a l l e y between L y t t o n and the S t e i n R i v e r . T e r r a c e s above the l e v e l of the main bench commonly occur at the mouths of t r i -b u t a r y streams. 3.4.1 G l a c i a l T i l l The l a r g e s t exposures of t i l l o ccur a l o n g the r i v e r banks at low e l e v a t i o n s where i t i s best seen beneath the broader s e c t i o n s of the main bench near the mouths of t r i b u t a r y c r e e k s , F i g . 3.24. The t i l l u s u a l l y i n c l u d e s a c o n s i d e r a b l e amount of s i l t as m a t r i x , but i t i s o t h e r w i s e v a r i a b l e r e g a r d i n g the s i z e and l i t h o l o g y of i n c l u d e d p a r t i c l e s . I t appears t h a t an i r r e g u l a r and d i s c o n t i n u o u s l a y e r of t i l l , up to 200' i n t h i c k n e s s , i s the l o w e s t and o l d -e s t Quaternary m a t e r i a l i n t h i s p a r t of the v a l l e y . T h i s i s thought t o be the b a s a l t i l l of the l a s t major g l a c i a t i o n . T i l l i s a l s o exposed at h i g h e r e l e v a t i o n s a l o n g the v a l l e y s i d e s , above the l e v e l of the main bench i n r o a d and stream c u t t i n g s . At H u l l Arden Creek the road c u t t i n g exposure shows 10' of t i l l composed of sub-rounded and s ub-angular p e b b l e s s e t i n a s i l t - c l a y m a t r i x . I t i s d i r e c t l y o v e r l a i n by the mudflow g r a v e l s of the H u l l Arden f a n . S i m i l a r exposures were found a d j a c e n t to many o t h e r e a s t bank t r i b u t a r i e s w i t h the t i l l commonly r e s t i n g d i r e c t l y upon bedrock. In a d d i t i o n , F i g u r e 3.24. I n k o i k o Fan, s o u t h e r n l i m b ; view southwards showing s i l t y t i l l near r i v e r l e v e l . 102 numerous o t h e r exposures show a s i l t y g r a v e l but s i n c e they have been graded or obscured by s l o p e wash, i t i s not c e r t a i n whether t h i s i s t i l l or some form of bedded m a t e r i a l . E v i d e n c e f o r the o c c u r r e n c e of two d i s t i n c t g l a c i a l s t a g e s was found at two l o c a t i o n s . In a road c u t t i n g near S p i n t l u m Creek, t i l l i s exposed o v e r l y i n g a sequence of bedded s i l t , sand and g r a v e l t h a t has been c o n t o r t e d and f a u l t e d by the weight and movement of the o v e r r i d i n g i c e , F i g . 3.25. About one h a l f m i l e n o r t h of H u l l Arden Creek t i l l i s exposed o v e r l y i n g cross-bedded s i l t and sand. S i n c e i t i s u n l i k e l y t h a t the bedded m a t e r i a l i n these exposures i s p r e g l a c i a l , then the presence o f / t h e o v e r l y i n g t i l l would i n d i c a t e t h a t at l e a s t one i n t e r g l a c i a l p e r i o d o c c u r r e d h e r e , a l t h o u g h t h e r e i s no i n d i c a t i o n as to i t s l e n g t h or r e l a t i v e i m p o r t a n c e . P o s s i b l y t h i s t i l l i s r e l a t e d t o a l a t e stage of g l a c i a t i o n c o r r e s p o n d i n g to the pebble t i l l i n the P a v i l i o n a r e a or the l a t e readvance at Texas Creek. 3 . 4-. 2 I c e - M a r g i n a l F e a t u r e s D e p o s i t s of g l a c i o - l a c u s t r i n e and f l u v i o - g l a c i a l o r i g i n o c cur a l o n g the western s i d e of the v a l l e y above the main bench. Very o f t e n these f e a t u r e s have been so s e v e r e l y eroded t h a t none of t h e i r o r i g i n a l topography remains and they are r e p r e s e n t e d o n l y by g r a v e l o u t c r o p p i n g s a l o n g the s t e e p l y s l o p i n g v a l l e y s i d e s . Such an exposure i s l o c a t e d above the main bench on the n o r t h e r n s i d e of I n k o i k o Creek and c o n s i s t s of a g r e a t (undetermined) t h i c k n e s s of - ' t o n - s o r t e d , F i g u r e 3.25. S p i n t l u m Creek exposure: show-i n g t i l l r e s t i n g upon d i s t u r b e d f l u v i o -g l a c i a l (?) s i l t and g r a v e l ; normal f a u l t i n g shown c l o s e - u p below. 104 m o d e r a t e l y bedded and cross-bedded g r a v e l s . E x t r e m e l y l a r g e a n g u l a r b o u l d e r s up to 12' i n diameter occur w i t h i n these g r a v e l s and must have .been d e p o s i t e d d i r e c t l y from the i c e . Whether or not these g r a v e l s were d e p o s i t e d d u r i n g the f i n a l m e l t i n g of the i c e , or whether they were d e p o s i t e d e a r l i e r and then eroded d u r i n g the l a s t g l a c i a l phase cannot be determined from evidence p r e s e n t l y a v a i l a b l e . Numerous p o c k e t s of g r a v e l and sand showing v a r i o u s p a t t e r n s of bedding i n d i c a t i v e of d e l t a i c and s h a l l o w water d e p o s i t i o n are common i n s m a l l r o a d c u t t i n g s a l o n g the western v a l l e y s i d e above the main bench. D e p o s i t i o n i n m a r g i n a l channels or m a r g i n a l l a k e s i s suggested f o r t h e i r o r i g i n . Other p o s s i b l e i c e - c o n t a c t or i c e - m a r g i n a l f e a t u r e s are t e r r a c e s o c c u r r i n g above the main bench and l o c a t e d i n embayments i n the v a l l e y s i d e s at the mouths of c r e e k s . These are most common al o n g the western v a l l e y s i d e , p a r t i -c u l a r l y • a t R i l e y , Towinock and Siwhe C r e e k s , see F i g . 3.14. Along the e a s t e r n v a l l e y s i d e are a number of i s o l a t e d s e d imentary b o d i e s b e l i e v e d t o have been d e p o s i t e d under i c e m a r g i n a l c o n d i t i o n s . They c o n t a i n a much h i g h e r p r o p o r t i o n of s i l t than i n e q u i v a l e n t l o c a t i o n s on the western bank. P o s s i b l y the m a r g i n a l l a k e s were deeper here and c u r r e n t s of lower v e l o c i t i e s ; i f the i c e s u r f a c e s l o p e d downwards from west t o e a s t , as suggested e a r l i e r ( c f . 2.2.2), t h i s i s l i k e l y . The p r o p o r t i o n of g r a v e l w i t h i n e a s t e r n sediments i n c r e a s e s n o t i c e a b l y towards streams where s m a l l d e l t a s were formed. The s i l t i s s i m i l a r i n appearance and t e x t u r e t o 105 the L i l l o o e t S i l t ( c f . 3.2.1; Appendix #1, S27, S28 and S29). 3.4.3 A g g r a d a t i o n a l G r a v e l s beneath the Main Bench Major a g g r a d a t i o n of the v a l l e y took p l a c e from both l o n g i t u d i n a l and l a t e r a l s o u r c e s , the F r a s e r R i v e r b e i n g the l o n g i t u d i n a l source and the numerous t r i b u t a r i e s the l a t e r a l . The F r a s e r g r a v e l s are s i m i l a r to those d e s c r i b e d above f o r the P a v i l i o n area ( c f . 3.3.1, u n i t s 2 and 4 ) . They c o n s i s t of w e l l rounded peb b l e s and c o b b l e s t h a t are u s u a l l y h o r i -z o n t a l l y bedded a l t h o u g h c r o s s - b e d d i n g i s not uncommon; c o a r s e r beds of b o u l d e r s occur at v a r y i n g v e r t i c a l i n t e r v a l s of between 50' and 150'. A g g r a d a t i o n from l a t e r a l s o u r c e s was c h i e f l y due to the c o n s t r u c t i o n of a l l u v i a l fans by t r i b u t a r y streams. These c o n s i s t of a g r e a t v a r i e t y of m a t e r i a l i n c l u d i n g s t r e a m - l a i d gravel's, mudflow g r a v e l s , sand and s i l t . The m a t e r i a l i s g e n e r a l l y w e l l bedded and d i p s between 2° and 10° towards the c e n t e r of the v a l l e y . The s t r u c t u r e and s t r a t i -graphy of t h i s m a t e r i a l are d i s c u s s e d In g r e a t e r d e t a i l i n S e c t i o n 10.1.2. The most prominent l a n d f o r m r e s u l t i n g from l a t e r a l a g g r a d a t i o n i s the broad s e c t i o n of the main bench t h a t extends a p p r o x i m a t e l y 6 m i l e s both n o r t h and south from L a l u w i s s e n Creek. The c o n s i d e r a b l e t r a n s v e r s e g r a d i e n t of t h i s f e a t u r e g i v e s a g r e a t d i f f e r e n c e i n h e i g h t between the f r o n t and back of t h i s bench, F i g . 3.14. A g u l l y y exposure beneath the bench shows t h a t i t i s composed almost e n t i r e l y 106 of mudflow g r a v e l s from l a t e r a l a g g r a d a t i o n ( a t l e a s t f o r the upper 200'), F i g . 10.10. The a n g u l a r u n c o n f o r m i t y between the upper, g e n t l y s l o p i n g beds and the l o w e r , more s t e e p l y s l o p i n g beds i m p l i e s t h a t the l a t t e r were p a r t i a l l y eroded and t r u n c a t e d as a r e s u l t of the l a t e r a l m i g r a t i o n of e i t h e r the F r a s e r or the t r i b u t a r y d u r i n g the a g g r a d a t i o n a 1 phase. G e n e r a l l y , i t appears t h a t d e p o s i t i o n from l a t e r a l and l o n g i t u d i n a l s o u r c e s took p l a c e s i m u l t a n e o u s l y d u r i n g most of the a g g r a d a t i o n beneath the c e n t r a l p o r t i o n of the v a l l e y . Here, t h e s e m a t e r i a l s are commonly found i n t e r b e d d e d , a l t h o u g h the r e l a t i v e p r o p o r t i o n of l a t e r a l ( f a n ) m a t e r i a l i n c r e a s e s towards the v a l l e y s i d e s , F i g . 3.26. A l s o , beds of l a c u s t r i n e s i l t and sand are common at a l l l e v e l s w i t h i n the a g g r a d a t i o n a l g r a v e l s , i n c l u d i n g the a l l u v i a l f a n s , i n d i c a t i n g t h a t temporary l a k e s were a common f e a t u r e of the v a l l e y at t h i s t i m e . Presumably, these formed at times when r a p i d l a t e r a l aggrada-t i o n had b l o c k e d ( o r p a r t i a l l y b l o c k e d ) the main v a l l e y and the F r a s e r was ponded f o r a s h o r t t i m e . To r e c o n s t r u c t the c o n d i t i o n s under which d e p o s i t i o n took p l a c e : l a t e r a l a g g r a d a t i o n was dominant at the mouths of the l a r g e r t r i b u t a r i e s w h i l s t d e p o s i t i o n by the F r a s e r took p l a c e p r i m a r i l y i n d i s c o n n e c t e d " b a s i n s " between the f a n s . Thus the growth of a f a n outwards and upwards would con-s t r i c t the main r i v e r and c o n f i n e i t a g a i n s t the o p p o s i t e v a l l e y s i d e (a p a t t e r n s t i l l i n evidence at the p r e s e n t t i m e ) . I f f a n d e p o s i t i o n were a g r a d u a l and c o n t i n u o u s p r o -c e s s , then p o s s i b l y the r i v e r c o u l d m a i n t a i n a r e g u l a r c h a n n e l THE FRASER VALLEY DURING POST-GLACIAL AGGRADATION View westwards and longitudinal section FIGURE 3.27 108 by c o n t i n u o u s e r o s i o n from the f a n t o e . However, i f f a n d e p o s i t i o n took p l a c e a t i r r e g u l a r i n t e r v a l s of time and by l a r g e volumes of d e b r i s (as i s the case w i t h mudflow), then i t i s p r o b a b l e t h a t the v a l l e y would be e n t i r e l y b l o c k e d f o r a s h o r t p e r i o d of time r e s u l t i n g i n some l a c u s t r i n e d e p o s i -t i o n . T h i s p a t t e r n f o r a g g r a d a t i o n has been r e c o n s t r u c t e d i n F i g . 3.27. C o n s e q u e n t l y , the c o n s t r u c t i o n a l s u r f a c e would not have a c o n s t a n t d o w n - v a l l e y g r a d i e n t , but would be i r r e g u -l a r , t e n d i n g to be more g e n t l e i n the s t r e t c h e s between the fans and s t e e p e n i n g l o c a l l y a c r o s s them. Fan b u i l d i n g by the t r i b u t a r i e s c o n t i n u e d f o r a r e l a t i v e l y s h o r t time a f t e r d e p o s i t i o n by the F r a s e r c e a s e d , so t h a t the c o n s t r u c t i o n a l s u r f a c e has the morphology of a l l u v i a l f a n s . During the most r e c e n t p e r i o d of d o w n c u t t i n g by the F r a s e r R i v e r , the i n n e r v a l l e y of the F r a s e r was formed, the main bench and most fans were d i s s e c t e d , and the low l e v e l r i v e r t e r r a c e s were cut by e r o s i o n of the u n c o n s o l i d a t e d g r a v e l s . 3.5 Chronology f o r the F r a s e r V a l l e y between L y t t o n and  P a v i l i o n A t e n t a t i v e c h r o n o l o g y f o r t h i s s e c t i o n of the F r a s e r V a l l e y i s g i v e n i n Table 3.1 and d i s c u s s e d below. The sequence of events o u t l i n e d i s the s i m p l e s t one t h a t c o u l d be d e v i s e d on the b a s i s of a v a i l a b l e e v i d e n c e . For t h a t p a r t of the v a l l e y between the B r i d g e R i v e r and P a v i l i o n a s e r i e s of events i s p o s t u l a t e d based upon the i n f o r m a t i o n g i v e n i n (3.2) above: TABLE 3.1 CHRONOLOGY FOR THE FRASER VALLEY s outh L y t t o n T exa s• Creek L i l l o o e t Bas i n B r i d g e R i v e r n o r t h P a v i l i o n -a l l u v i a l f a n s a g g r a d a t i o n i c e r e t r e a t i c e (6) Texas and Jones bedrock gorges d o w n c u t t i n g and t e r r a c i n g L i l l o o e t ' ' t e r r a c e s (5) G l a c i a l Lake L i l l o o e t ( L i l l o o e t s i l t ) ( S eton d e l t a ) ( F o u n t a i n d e l t a ) ( 4) R i v e r l a n d s kame t e r r a c e downcutt i n g commenc ed (4) d e c a y i n g pebble t i l l i c e ( F o u n t a i n kame) ( e a r t h f l o w ) (mudflow) i c e r e t r e a t i c e i c e (3) Pebble t i l l g l a c i a t i o n ( p ebble t i l l ) i c e advance (sand and d e l t a i c g r a v e l ) (2) Major a g g r a d a t i o n w i t h s h o r t degrad-a t i o n i n middl e i c e i c e i c e r e t r e a t (1) L a c u s t r i n e c o n d i t i o n s ( " s t r i p e d " s i l t and g r a v e l ) 110 (1) The " s t r i p e d " s i l t and g r a v e l t h a t i s l o c a t e d n o r t h of P a v i l i o n and S l o k Creeks appears to be the o l d e s t s e d i -mentary u n i t of the Quaternary t h a t i s r e p r e s e n t e d here. A l t h o u g h t h i s was not c l o s e l y examined, i t may be supposed t h a t the i n t e r b e d d i n g of s i l t w i t h g r a v e l r e s u l t e d from e i t h e r a l t e r n a t i n g l a c u s t r i n e and f l u v i a l c o n d i t i o n s , or . the f l u c t u a t i n g depth of a l a k e . In ei'ther c a s e , i t i s most l i k e l y t h a t these c o n d i t i o n s were c o n t r o l l e d by i c e i n the v a l l e y f u r t h e r downstream. S i n c e t h i s m a t e r i a l extends up to almost 2 , 000' , t h i s f i g u r e g i v e s the minimum p o s s i b l e e l e v a t i o n f o r the l a k e s u r f a c e and the i c e dam. The dam may have been l o c a t e d as f a r n o r t h as P a v i l i o n and Slok Creeks (which i s a p p a r e n t l y the s o u t h e r n l i m i t of t h i s m a t e r i a l ) , or at any p o i n t f a r t h e r s o u t h , F i g . 3.28. The anomalous sequence of " s t r i p e d " s i l t and g r a v e l t h a t i s l o c a t e d beneath the main bench j u s t south of the bedrock s e c t i o n (see 3.2) may p o s s i b l y be p a r t of t h i s e a r l i e r m a t e r i a l . I t stands i s o l a t e d from the main body of sediment to the n o r t h due e i t h e r t o h a v i n g been d e p o s i t e d i n a s m a l l e r g l a c i a l l a k e l o c a t e d on the e a s t e r n s i d e of the i c e l o b e t h a t o c c u p i e d most of the v a l l e y , or to e r o s i o n f o l l o w i n g d e p o s i -t i o n which removed the i n t e r m e d i a t e body of sediment. (2) The next youngest body of sediment i n t h i s p a r t of the v a l l e y i s the a g g r a d a t i o n a l g r a v e l s t h a t are s i t u a t e d beneath the main bench. S i n c e t h e s e occupy the c e n t r a l p o r t i o n of the v a l l e y and extend upwards from the bedrock to j u s t below the the s u r f a c e of the main bench, i t must be supposed t h a t e i t h e r 112 the " s t r i p e d " s i l t and g r a v e l ( ( 1 ) above) was never d e p o s i t e d h e r e , or t h a t i t was eroded and removed p r i o r t o t h i s a g g r a d a t i o n . The l a t t e r s i t u a t i o n i s perhaps the l e a s t l i k e l y , s i n c e i f e r o s i o n took p l a c e , t h e r e i s . no a'dteq^ att-re e x p l a n a t i o n as t o why i t o c c u r r e d o n l y downstream from P a v i l i o n and Slok C reeks. So i t i s t e n t a t i v e l y assumed t h a t the " s t r i p e d " m a t e r i a l was not d e p o s i t e d i n those p a r t s of the v a l l e y now o c c u p i e d by the main bench g r a v e l s , and t h a t t h i s s e c t i o n of the v a l l e y was. o c c u p i e d by an i c e lob e d u r i n g d e p o s i t i o n o f the s t r i p e d sequence. With the m e l t i n g of t h i s i c e l a c u s t r i n e c o n d i t i o n s ended and a g g r a d a t i o n of the main bench g r a v e l s commenced. A p o s s i b l e pause i n a g g r a d a t i o n i s i n d i c a t e d by the coa r s e g r a v e l bed w h i c h , as was e a r l i e r suggested ( 3 . 3 . 1 ) , may be a l a g d e p o s i t r e s u l t i n g from a phase of d o w n c u t t i n g . T h i s would be c o n t r o l l e d by changes i n base l e v e l f u r t h e r downstream, p o s s i b l y i n the L i l l o o e t b a s i n . I t i s not known how f a r downstream the a g g r a d a t i o n a l g r a v e l s o r i g i n a l l y extended, s i n c e they are absent (and r e p l a c e d , by the pebble t i l l ) downstream from F o u n t a i n Creek. I t may be supposed e i t h e r t h a t they were eroded from t h i s s e c t i o n of the v a l l e y by the pebble t i l l i c e (3.. 3.2), or t h a t the i c e o c c u p i e d t h i s s e c t i o n of the v a l l e y d u r i n g t h e i r d e p o s i t i o n . The d o t t e d l i n e (1) on F i g . 3.14- i n d i c a t e s the r e c o n s t r u c t e d l o n g i t u d i n a l p r o f i l e of the main bench h e r e , and'shows i t t e r m i n a t i n g at the kame t e r r a c e s between 42 and 47 m i l e s n o r t h of L y t t o n , t h a t i s , between the L i l l o o e t 113 b a s i n and F o u n t a i n Creek. P o s s i b l y , these kame t e r r a c e s were a s s o c i a t e d w i t h the i c e margin d u r i n g t h i s phase of aggrada-t i o n , a l t h o u g h the same t e r r a c e s may have been formed by the de c a y i n g pebble t i l l i c e , see below. S i n c e the a g g r a d a t i o n a l m a t e r i a l below the main bench c o n s i s t s c h i e f l y of g r a v e l , i t may be supposed t h a t d e p o s i t i o n was by the r i v e r a t t h i s time ( r a t h e r than l a c u s t r i n e ) . T h i s i m p l i e s t h a t d r a i n a g e was a b l e t o bypass any i c e r e m a i n i n g i n the v a l l e y at an e l e v a t i o n of l e s s than 1,250' (which i s the h e i g h t of the downstream end of the main bench), s u g g e s t i n g i n t u r n , t h a t the F r a s e r V a l l e y must have been p a r t i a l l y f r e e of i c e at t h i s t i m e , F i g . 3.29. (3) The d e p o s i t i o n of a t h i n l a y e r of pebble t i l l upon the s u r f a c e of the main bench was the next n o t a b l e event. T h i s r e q u i r e d a readvance of the i c e to at l e a s t as f a r n o r t h as P a v i l i o n . D u r i n g the advance of the i c e , the r i v e r was ponded and l a c u s t r i n e sand and g r a v e l d e p o s i t e d p r i o r to the pebble t i l l . S i n c e i t i s thought t h a t i c e was s u p p l i e d f o r t h i s readvance from the a c c u m u l a t i o n zone of the Coast Range to the west and south of L i l l o o e t , the downstream p a r t of the F r a s e r V a l l e y i n c l u d i n g the L i l l o o e t b a s i n must have been o c c u p i e d by i c e at t h i s s t a g e , see Table 3.1, F i g . 3.30. -(4) The decay of the pebble t i l l i c e was accompanied by the c o n s t r u c t i o n of kame t e r r a c e s i n the s t r e t c h of v a l l e y between F o u n t a i n s t a t i o n and the B r i d g e R i v e r . These g r a v e l s and t h e i r a s s o c i a t e d benches can be t r a c e d from here t o the n o r t h e r n end of the L i l l o o e t b a s i n where the g r a v e l s .appear 115 t o be c o n t i n u o u s w i t h the F o u n t a i n d e l t a of G l a c i a l Lake L i l l o o e t . T h i s would i n d i c a t e t h a t the g l a c i a l l a k e came i n t o e x i s t e n c e sometime d u r i n g the decay of the pebble t i l l i c e . Thus a s t a g n a t i n g , i s o l a t e d b l o c k of i c e remained i n the "S" bend s e c t i o n of the v a l l e y ; d r a i n a g e from the v a l l e y f u r t h e r n o r t h f l o w e d over or around t h i s b l o c k a g e s i n c e t h e r e i s no e vidence t h a t any l a k e f o l l o w e d the pebble t i l l g l a c i a t i o n , F i g . 3.31. F o l l o w i n g the d e p o s i t i o n of the pebble t i l l , f l u v i a l and mudflow d e p o s i t i o n m o d i f i e d the s u r f a c e of the main bench s l i g h t l y . E a r t h f l o w m a t e r i a l i n t e r b e d d e d w i t h r i p p l e marked sand near F o u n t a i n and Gibbs Creeks i n d i c a t e s a s m a l l amount of f l u v i a l ( o r very s h a l l o w l a c u s t r i n e ) d e p o s i t i o n f o l l o w e d the i c e r e t r e a t . D i s s e c t i o n of the c o n s t r u c t i o n a l s u r f a c e t o form the i n n e r v a l l e y of the F r a s e r commenced w i t h the m e l t i n g , o f i c e at the F o u n t a i n kame and the consequent l o w e r i n g of the l o c a l base l e v e l . Then the new base l e v e l would be formed by G l a c i a l Lake L i l l o o e t and m a i n t a i n e d d u r i n g the e x i s t e n c e of the l a k e . P o s s i b l y the narrow t e r r a c e on the o u t e r edge of the main bench i n the F o u n t a i n - P a v i l i o n area was formed d u r i n g t h i s p e r i o d of s t a b l e base l e v e l . Subsequent d o w n c u t t i n g took p l a c e a f t e r the d r a i n a g e of the l a k e c o r r e s p o n d i n g w i t h d o w n c u t t i n g throughout the l e n g t h of the v a l l e y . The L i l l o o e t b a s i n was the f i r s t p a r t of the F r a s e r V a l l e y a d j a c e n t to the Coast Range to become i c e f r e e . Events here were as f o l l o w s : 116 (4-, c o n t i n u e d ) . As i c e began to t h i n i n the L i l l o o e t b a s i n , m e l t w a t e r f l o w e d i n t o the d e p r e s s i o n and g r a v e l was d e p o s i t e d i n c o n t a c t w i t h the i c e . As f u r t h e r m e l t i n g took p l a c e , the f i r s t break i n the v a l l e y - f l o o r i c e - c o v e r appeared i n the approximate p o s i t i o n of the R i v e r l a n d s kame t e r r a c e . G r a v e l d e p o s i t i o n c o n t i n u e d here f o r some time a f t e r the i c e l o b e s had s e p a r a t e d and y e t remained i n c o n t a c t w i t h the kame to the n o r t h and south w h i l e the h i g h rims on t h i s f e a t u r e were c o n s t r u c t e d , F i g . 3.31. Thus at t h i s stage t h e r e were t h r e e i c e f r o n t s b o r d e r i n g the i c e - f r e e a r e a ; the F r a s e r and B r i d g e R i v e r i c e ( o f the pebble t i l l g l a c i a t i o n ) t o the n o r t h , the F r a s e r i c e to the s o u t h , and i c e t o the east i n Seton V a l l e y . (5) With f u r t h e r r e t r e a t of the t h r e e i c e f r o n t s , most of the L i l l o o e t b a s i n was uncovered and G l a c i a l Lake L i l l o o e t formed by the ponding of mel t w a t e r here. The t h i c k s i l t d e p o s i t s are the major ev i d e n c e f o r the e x i s t e n c e of t h i s l a k e ; no s t r a n d l i n e s have been found. Thus the minimum h e i g h t of the s u r f a c e of the l a k e can o n l y be e s t i m a t e d i n d i r e c t l y by the h i g h e s t e l e v a t i o n at which s i l t d e p o s i t s have been foun d , a p p r o x i m a t e l y 1,000'. The absence of s t r a n d l i n e s i n the L i l l o o e t b a s i n may be e x p l a i n e d on s e v e r a l a c c o u n t s : f i r s t l y , the v a l l e y s i d e s are very steep h e r e , and any narrow s t r a n d l i n e s would be q u i c k l y o b l i t e r a t e d by p o s t - l a k e s l o p e p r o c e s s e s ; s e c o n d l y , i f the l e v e l of the g l a c i a l l a k e was subject, to f l u c t u a t i o n s (as i s l i k e l y , s i n c e the dam was i c e ) , s t r a n d l i n e s may never 117 have had time t o f u l l y develop ( p a r t i c u l a r l y on the bedrock v a l l e y w a l l s ) ; t h i r d l y , s t r a n d l i n e s may have been b u r i e d by younger c o l l u v i u m or a g g r a d a t i o n a l g r a v e l s . As G l a c i a l Lake L i l l o o e t expanded, the g r a v e l s of the F o u n t a i n d e l t a were d e p o s i t e d a l o n g i t s n o r t h e r n m a r g i n , at f i r s t w i t h the edge of the i c e submerged beneath the l a k e as suggested by the c h a r a c t e r of the g r a v e l s here ( 3 . 2 . 3 ) , but l a t e r , as the edge of the i c e r e t r e a t e d upstream, i t i s l i k e l y t h a t a b r a i d e d , a g g r a d i n g , g r a v e l stream connected the i c e snout w i t h the l a k e . Two m i l e s n o r t h of the l a k e the r e t r e a t i n g i c e l o b e s p l i t i n t o a s t agnant F r a s e r V a l l e y i c e b l o c k (the remnant pebble t i l l i c e ) and a B r i d g e R i v e r l o b e . As t h i s l a t t e r r e t r e a t e d upstream, a g g r a d a t i o n commenced i n the B r i d g e R i v e r V a l l e y . The Seton D e l t a was b u i l t i n t o t h a t p o r t i o n of the l a k e o c c u p y i n g the Seton r e - e n t r a n t , w h i l s t the i c e l o b e p r o b a b l y o c c u p i e d the b a s i n of the p r e s e n t Seton Lake. The l o c a t i o n of the i c e b l o c k a g e at the s o u t h e r end of G l a c i a l Lake L i l l o o e t has not been e s t a b l i s h e d . The s i l t extends south to one m i l e beyond the mouth of Texas Creek, and so i t i s l i k e l y t h a t the southernmost p o s i t i o n of the dam was here. Another p o s s i b i l i t y t h a t s h o u l d be c o n s i d e r e d i s t h a t s e v e r a l s m a l l e r l a k e s may have e x i s t e d i n the v a l l e y r a t h e r than one l a r g e r l a k e . The s i l t exposures are not c o n t i n u o u s a l o n g the v a l l e y but are broken i n t o t h r e e s e c t i o n s : (1) L i l l o o e t b a s i n ; (2) Wick Creek to R i l e y Creek; (3) i n the v i c i n i t y o f Texas Creek. S i n c e the e a s t e r n margin 118 o f the Jones bench r i s e s above the supposed l a k e l e v e l ( F i g . 3.14) and i t i s proposed t h a t the western p a r t of the.bench was o c c u p i e d by i c e at t h i s time (see (3) b e l o w ) , t h i s area of h i g h ground would have s e p a r a t e d a l a k e i n the L i l l o o e t b a s i n from the a d j a c e n t one to the s o u t h . So G ".. ... .1 e;a s t .* c.- '. two s e p a r a t e l a k e s p r o b a b l y d i d e x i s t . D u r i n g the e x i s t e n c e of the g l a c i a l l a k e s d r a i n a g e was down t h e . F r a s e r V a l l e y , e i t h e r v i a m a r g i n a l channels between i c e and v a l l e y s i d e , or over the s u r f a c e of the i c e , F i g . 3.32. (6) C u t t i n g of bedrock gorges was commenced by the downvalley d r a i n a g e . P o s s i b l y i c e and g l a c i a l t i l l o c c u p y i n g the western s i d e of the Jones bench were s u f f i c i e n t t o d i v e r t the r i v e r a l o n g the e a s t e r n margin of t h i s f e a t u r e . A s l i g h t readvance of i c e from the b a s i n of Texas Creek i n to the F r a s e r V a l l e y formed the kame and m o r a i n e - l i k e f e a t u r e s on the Texas bench and c o n f i n e d the r i v e r a g a i n s t the e a s t e r n v a l l e y w a l l where a gorge through bedrock was i n i t i a t e d . The d r a i n a g e of the l a k e i n the L i l l o o e t b a s i n and subsequent t e r r a c i n g of the L i l l o o e t s i l t was c o n t r o l l e d by the deepening of the Jones bedrock gorge. The L i l l o o e t t e r r a c e s and the t e r r a c e s w i t h i n the Seton r e - e n t r a n t r e s u l t e d from the r e l a t i v e l y g r e a t l a t e r a l m i g r a t i o n , but c o m p a r a t i v e l y slow d o w n c u t t i n g , of the s e r i v e r s d u r i n g the f o r m a t i o n of the bedrock gorge, F i g . 3.33. W i t h i n t h a t p a r t of the F r a s e r V a l l e y t o the south of Texas Creek a g g r a d a t i o n commenced as soon as i c e melted from 120 the v a l l e y f l o o r . A g g r a d a t i o n by both the F r a s e r R i v e r and t r i b u t a r y streams b u r i e d and/or p a r t i a l l y eroded the b a s a l t i l l and b u i l t up an i r r e g u l a r c o n s t r u c t i o n a l s u r f a c e c o n s i s t i n g i n p a r t of an a g g r a d i n g r i v e r bed and i n p a r t of a l l u v i a l f a n s . F i n a l l y , a p e r i o d of d o w n c u t t i n g by the F r a s e r d i s -s e c t e d the c o n s t r u c t i o n a l s u r f a c e t o form the main bench and the lower r i v e r t e r r a c e s . CHAPTER IV THOMPSON VALLEY--DEVELOPMENT OF CONTEMPORARY LANDFORMS 4 . 1 I n t r o d u c t i o n Between A s h c r o f t and Spences B r i d g e the 4,000' deep Thompson V a l l e y c u t s through the Thompson P l a t e a u , a sub-d i v i s i o n of the I n t e r i o r P l a t e a u ( H o l l a n d , 1964), F i g . 4.1. The v a l l e y i s u n d e r l a i n by a v a r i e t y of g e o l o g i c a l f o r m a t i o n s r a n g i n g i n age from Permian to l a t e - T e r t i a r y , w i t h the course of the Thompson R i v e r a p p r o x i m a t i n g the boundary l i n e between the g r a n i t i c r o c k s of the Guichon Creek b a t h o l i t h t o the east and v o l c a n i c and sedimentary r o c k s to the west ( D u f f e l l and McTaggart, 1952), F i g . 4.2. Alt h o u g h s t r a t i g r a p h i c e v idence has been found of a P l e i s t o c e n e i n t e r g l a c i a l p e r i o d , i t i s not y e t s u f f i c i e n t t o i n d i c a t e the d u r a t i o n or importance (Dawson, 1896; D r y s d a l e , 1914; c f . 1.2.2). Thus the i n t e r g l a c i a l beds may e i t h e r r e p r e s e n t an i n t e r s t a d i a l or o n l y a phase of the g e n e r a l d e g l a c i a t i o n . Over much of the upland s u r f a c e a veneer of g l a c i a l t i l l remains to the p r e s e n t t i m e , but w i t h i n the v a l l e y s , i f much of t h i s m a t e r i a l i s p r e s e n t , i t has been b u r i e d by l a t e r l a c u s t r i n e and f l u v i a l d e p o s i t s and i s r a r e l y exposed. I s o l a t e d areas of m o r a i n i c topography o c c u r , as at an e l e v a t i o n of 1,600' between Pimainus and I n k i k u h Creeks and THOMPSON VALLEY LOCATION MAP FIGURE 4.1 4 0 4 8 12 1 1 1 1 1 1 • • Mi les FIGURE 4.2 THOMPSON VALLEY - BEDROCK GEOLOGY LEGEND TERTIARY Miocene or Earlier KAMLOOPS GROUP (11,12) 12 Basalt,'andesite and rhyolite-, associated tuffs and breccias I I COLDWATER BEDS (? ) ; sandstone, shale and conglomerate, coal CRETACEOUS or TERTIARY 10 Conglomerate, sandstone and shale CRETACEOUS Lower Cretaceous - KINGSVALE GROUP Basalt and andesite-, agglomerate, tu f f and breccia 8 Arkose, conglomerate, shale and greywacke SPENCES BRIDGE GROUP Andesite, dacite, basalt and rhyolite; tuff, breccia and agglomerate, JURASSIC sandstone, greywacke, arkose and conglomerate Middle and Upper Jurassic 6 Shale, conglomerate and sandstone TRIASSIC Upper Triassic NICOLA GROUP -5 Basalt and andesite, tuff and agglomerate; limestone, quartzite, PERMIAN and ( ? ) argillite, greywacke and arkose EARLIER CACHE CREEK GROUP 4 MARBLE CANYON FORMATION : limestone 3 Greenstone; chert, argillite, minor limestone and quartzite; chlorite INTRUSIVE ROCKS and quartz-mica schist JURASSIC or CRETACEOUS Lower Cretaceous or Earlier 2 MOUNT LYTTON BATHOLITH ; Granodiorite, quartz-diorite and diorite JURASSIC Lower Jurassic l GUICHON CREEK BATHOLITH: Granite, granodiorite, quartz-diorite and diorite After S. Duffel l and K. C. McTaggart FIGURE 4 .2 124 to the west of Red H i l l , F i g . .4.3. In both t h e s e c a s e s , the t i l l i s p o s s i b l y r e s t i n g , upon i n t e r g l a c i a l benches. 4 . 2 L a c u s t r i n e D e p o s i t i o n The f i r s t p o s t - G l a c i a l d e p o s i t i o n i n the v a l l e y took p l a c e i n g l a c i a l Lake Thompson ( c f . 2.2.1) and commenced imm e d i a t e l y f o l l o w i n g i c e r e t r e a t . E a r l y d e p o s i t i o n took p l a c e i n m a r g i n a l l a k e s , i n c o n t a c t w i t h an i c e l o b e t h a t s t i l l o c c u p i e d the c e n t r a l p o r t i o n of the v a l l e y . E v i d e n c e f o r t h i s phase comes from d e l t a i c g r a v e l s and s i l t at the mouths of I n k i k u h and Pimainus Creeks whose bedding was s e v e r e l y a f f e c t e d by slumping as the i c e m e l t e d ; subsequent h i g h e r d e p o s i t s are u n d i s t u r b e d . W i t h i n t h i s s e c t i o n of the Thompson V a l l e y a c l e a r p a t t e r n may be r e c o g n i z e d r e g a r d i n g the d i s t r i b u t i o n and t y p e s of l a c u s t r i n e sediment. The most wides p r e a d d e p o s i t was of g l a c i o - l a c u s t r i n e s i l t , s i m i l a r i n many r e s p e c t s t o the s i l t b o d i e s of the Okanagan, South Thompson and F r a s e r ( L i l l o o e t a r e a , c f . 3.2.1) V a l l e y s . T h i s w i l l be r e f e r r e d to from now on as the Spences B r i d g e s i l t . L o c a l l y l a c u s t r i n e sands and g r a v e l s are i m p o r t a n t where t r i b u t a r y streams e n t e r e d the l a k e . 4.2.1 The Spences B r i d g e S i l t T h i s s i l t c omprises the c h i e f p o s t - G l a c i a l d e p o s i t i n the v a l l e y . I t i s c o n t i n u o u s from A s h c r o f t t o about 3 m i l e s s o u t h of Spences B r i d g e and a l s o extends f o r s e v e r a l m i l e s from Spences B r i d g e up the N i c o l a V a l l e y ( A n d e r t o n , i n LEGEND s f X r * terrace scarp — break of slope at back of terrace silt bench 7^/'V alluvial fans Twaal Creek ''•/f. \\J ///' U-O ( Oregon Jack Creek THOMPSON VALLEY - BENCHES, TERRACES AND ALLUVIAL FANS Pukaist Creek Coldstream Creek APPROXIMATE SCALE I 0 miles Map compiled from air photo interpretation - uncorrected FIGURE 4.3 126 p r e p a r a t i o n ) . A l t h o u g h t h e r e are numerous exposures of the s i l t at many l o c a t i o n s i n the v a l l e y , one i n p a r t i c u l a r was examined i n d e t a i l . I t i s l o c a t e d at T s i n g k a h t l e , 8 m i l e s to the n o r t h of Spences B r i d g e ( F i g . 4-. 1) where a 300' h i g h s e c t i o n of the s i l t and u n d e r l y i n g m a t e r i a l i s exposed i n the banks of a s m a l l r a v i n e . T h i s g i v e s the most complete r e c o r d a v a i l a b l e and i s b e l i e v e d t o show a l l the c h a r a c t e r -i s t i c f e a t u r e s of a f i n e - g r a i n e d l a c u s t r i n e sequence. The d e t a i l e d d e s c r i p t i o n of t h i s exposure t h a t f o l l o w s ' i s thought t o be r e p r e s e n t a t i v e of the Spences B r i d g e s i l t as a whole. The T s i n g k a h t l e Exposure The sequence of beds here i s l i s t e d i n Table 4.1 and t h e i r geometry i s shown d i a g r a m m a t i c a l l y i n F i g . 4.4. I t comprises 11 u n i t s which may be d i v i d e d i n t o t h r e e groups: the b a s a l g r a v e l , u n i t 11; l a m i n a t e d sand and s i l t , u n i t s 4, 5, 6 and 10 and s i l t , u n i t 2; c o a r s e r m a t e r i a l , u n i t s 1 , 3 , 7 , 8 and 9. The B a s a l G r a v e l ( U n i t 11) The T s i n g k a h t l e exposure g i v e s l i t t l e i n d i c a t i o n as to the i n t e r n a l s t r u c t u r e or mode of o r i g i n of t h i s bed, s i n c e the o u t c r o p has been obscured by slopewash. However, i t s presence i s c l e a r l y i n d i c a t e d by a g r a v e l veneer on the ground s u r f a c e which ends a b r u p t l y at the c o n t a c t w i t h the l a m i n a t e d sand, u n i t 10, Fig.- 4.5. T h i s c o n t a c t v a r i e s - i n TABLE ,4. 1 THE TSINGKAHTLE LACUSTRINE SEQUENCE Top of S e c t i o n T h i c k n e s s ( f e e t ) U n i t 1. W e l l rounded c o b b l e - g r a v e l . U n i t 2. White s i l t w i t h w e l l marked j o i n t s and bedding p l a n e s r e s u l t i n g i n b l o c k y appearance; i n c l u d e s c o n t o r t i o n s c o n s i s t i n g of r o l l e d up beds of t h i n l y l a m i n a t e d s i l t and f i n e sand w i t h p s e u d o - r i p p l e - m a r k s , up t o 6' dia m e t e r and surrounded by u n d i s t u r b e d s i l t , l o c a t e d 25' to 4-0' from upper s u r f a c e of s i l t . 12 150' U n i t 3. Lens? of n o n - s o r t e d and non-bedded g r a v e l w i t h s i l t y m a t r i x . U n i t 4. I n t e r l a m i n a t e d s i l t and f i n e sand w i t h r i p p l e i = m'arks . U n i t 5. I n t e r l a m i n a t e d s i l t and f i n e sand; l a m i n a t i o n s t h i n n e r upwards. U n i t 6. Laminated s i l t w i t h a l t e r n a t i n g l i g h t and dark bands; u n d u l a t i n g bedding p l a n e s ; p i n c h e s out eastwards. U n i t 7. F i n e bedded g r a v e l d i p p i n g eastwards at 15°, c o a r s e r pebble bed on t o p ; grades i n t o c l a y , s i l t and sand laminae to e a s t . 10 10 U n i t 8. W e l l compacted, massive medium-grained sand. U n i t 9. Rounded to s u b - a n g u l a r , non-bedded 1 g r a v e l w i t h much s i l t y m a t r i x ; s l i g h t d e crease i n p a r t i c l e s i z e upwards. U n i t 10. I n t e r l a m i n a t e d c o a r s e and f i n e sand g r a d i n g upwards i n t o s i l t and f i n e sand; t h i c k n e s s of laminae 1 / l O t h " to s e v e r a l i n c h e s ; i n c l u d e s many s m a l l - s c a l e p r i m a r y s t r u c t u r e s e s p e c i a l l y near base - 80 u n c o n f o r m i t i e s , c u t - a n d - f i l l , r i p p l e -marks, l o a d and flame and a l s o t h i n g r a v e l l e n s e s and s i l t - p e b b l e s ; many 100 Table 4.1 ( c o n t i n u e d ) „, j - 0 . . T h i c k n e s s Top of S e c t i o n , _ , , ( f e e t ) s m a l l normal f a u l t s w i t h downthrow l e s s than 6" to e a s t . U n i t 11. Rounded peb b l e s and c o b b l e s i n grey g r a v e l , bedding o b s c u r e d , u n d u l a t i n g upper s u r f a c e Bedrock FIGURE 4.4 THOMPSON VALLEY -TSINGKAHTLE LACUSTRINE SEQUENCE (l) TERRACE GRAVEL Contortions (2) WHITE SILT Lense of mudflow gravel Erratic boulders (3) MUDFLOW OR AN/EL (4) SILT, SAND LAMINATIONS (5) SILT, SAND LAMINATIONS (6) SILT LAMINATIONS (7) DELTAIC GRAVEL 8) MASSIVE SAND (9) MUDFLOW GRAVEL (10) SAND LAMINATIONS ( l l ) BASAL GRAVEL (12) BEDROCK V.E. 3 x interlaminated sand and silt F i g u r e 4.5. G e n e r a l view o f h i l l s i d e a d j a c e n t to T s i n g k a h t l e exposure showing the d i f f e r e n t i a t i o n of the t h r e e major s e d i m e n t a r y u n i t s by p a t t e r n e d ground s u r f a c e . F i g u r e 4.5. G e n e r a l view of h i l l s i d e a d j a c e n t to T s i n g k a h t l e exposure showing the d i f f e r e n t i a t i o n of the t h r e e major sedimentary u n i t s by p a t t e r n e d ground s u r f a c e . 13t e l e v a t i o n from p l a c e t o p l a c e , both at T s i n g k a h t l e and wherever e l s e i t i s exposed. For example, F i g . .4.6 i l l u s -t r a t e s the i n t e r p r e t a t i o n p l a c e d upon a s e r i e s of g r a v e l o u t c r o p s noted a l o n g a p r o f i l e c r o s s i n g the e a s t e r n v a l l e y -s i d e t o the south of Spatsum. Here, an i r r e g u l a r g r a v e l sur-f a c e i s thought to u n d e r l i e p o c k e t s of l a c u s t r i n e m a t e r i a l s which were f o r m e r l y much more e x t e n s i v e . S i n c e such an i r r e g u l a r upper s u r f a c e f o r the g r a v e l i s u n l i k e l y t o r e s u l t from e i t h e r f l u v i o - g l a c i a l or g l a c i o -l a c u s t r i n e o r i g i n , i t i s suggested t h a t the g r a v e l i s of l a t e - G l a c i a l o r i g i n , p r o b a b l y a b l a t i o n moraine dumped d i r e c t l y from, the body of s t a g n a t i n g i c e . The S i l t and the Laminated Sand and S i l t ( U n i t 2, U n i t s 4, 5, 6 and 10) These u n i t s are i n t e r p r e t e d as r e p r e s e n t i n g a s i n g l e and c o n t i n u o u s sequence of beds l a i d down i n a l a k e i n f r o n t of a r e t r e a t i n g i c e l o b e . The lowe s t l a m i n a t e d beds ( u n i t 10) w i t h the numerous p r i m a r y s t r u c t u r e s r e s u l t e d from l a c u s t r i n e d e p o s i t i o n i n c l o s e p r o x i m i t y t o the i c e and the r e c e i p t of m a t e r i a l c a r r i e d by p o w e r f u l d e n s i t y c u r r e n t s of v a r y i n g s t r e n g t h . These c u r r e n t s a l s o performed e r o s i o n i n l o c a l i z e d areas t o produce the minor u n c o n f o r m i t i e s , F i g . 4.7. The l o a d and flame s t r u c t u r e s ( F i g . 4.7) i n d i c a t e t h a t the whole mass was undergoing s e t t l i n g and compaction w h i l s t s t i l l s a t u r a t e d , p o s s i b l y due t o r a p i d a c c u m u l a t i o n of o v e r l y i n g sediment. A second p e r i o d of s e t t l i n g , PROFILE OF EASTERN SIDE OF THOMPSON VALLEY SHOWING THE INFERRED RELATIONSHIP OF SILT AND GRAVEL WEST FEET 8 0 0 -7 0 0 -6 0 0 5 0 0 400-1 3 0 0 2 0 0 -100-° basal gravel Thompson River E A S T mudflow gravel 2,000 3 .000 4 .000 FEET - observed contact between silt and gravel - hypothetical contact between silt and gravel LOCATION - 12 5 miles north of Spences Bridge, I mile north* of Spatsum VERTICAL EXAGGERATION 2 x 23 CD c JO m cn F i g u r e 4.7k. T s i n g k a h t l e exposure; i n t e r l a m -i n a t e d c o a r s e and f i n e sand i n c u r r e n t r i p p l e marks, U n i t 10; s c a l e 1:6. F i g u r e 4.7b. T s i n g k a h t l e exposure: l o a d and flame s t r u c t u r e s i n i n t e r l a m i n a t e d c o a r s e and f i n e sand; e r o s i o n a l u n c o n f o r m i t y above; s c a l e 1:6. 134 p r o b a b l y d u r i n g d r y i n g i s suggested by the numerous s m a l l normal f a u l t s , F i g - 4.8. These i n v a r i a b l y have the down-throw s i d e towards the main v a l l e y . The a l t e r n a t i n g c o a r s e and f i n e sand laminae i n d i c a t e p e r i o d i c v a r i a t i o n s i n the s t r e n g t h of c u r r e n t s c a r r y i n g m a t e r i a l to the l a k e . They are thought t o be due t o normal v a r i a t i o n s i n stream f l o w , r a t h e r than to s e a s o n a l f l u c t u a -t i o n s , s i n c e c o n s i d e r a b l e ; changes i n c u r r e n t s t r e n g t h .are a l s o i n d i c a t e d by the numerous e r o s i o n a l f e a t u r e s mentioned above. A l s o , many of the c o a r s e laminae i n u n i t 10 show graded b e d d i n g , i n d i c a t i n g d e p o s i t i o n as a r e s u l t of a s i n g l e f l o o d . The laminae g e n e r a l l y become t h i n n e r upwards , r e f l e c t -i n g d e p o s i t i o n a t an ever i n c r e a s i n g d i s t a n c e from the i c e f r o n t , F i g . 4.9. U n c o n f o r m i t i e s and c u ' t - a n d - f i l l s t r u c t u r e s are a l s o s c a r c e r upwards f o r the same r e a s o n , y e t some weak f l o w on the l a k e bed r e s u l t e d i n the f o r m a t i o n of c u r r e n t r i p p l e marks h i g h i n the l a m i n a t e d sequence at the base of the s i l t , F i g . ' 4,.. 10 . F i n a l l y , s i l t was d e p o s i t e d i n q u i e t waters at the g r e a t e s t d i s t a n c e from the i c e f r o n t . As a p o i n t of c omparison, i t may be noted t h a t the s i l t here does not show the obvious p a t t e r n of " v a r v e s " as d e s c r i b e d by F u l t o n (1965) f o r the South Thompson S i l t i n the Kamloops b a s i n . E i t h e r the mechanism t h a t produced, the v a r v e s , namely a s t r o n g summer i n f l u x of s i l t - l a d e n waters from a l a r g e , r e c e n t l y d e g l a c i a t e d watershed was not a c t i v e h e r e , or the F i g u r e 4.8. T s i n g k a h t l e exposure: i n t e r l a m i n a t e d s i l t and f i n e sand w i t h s m a l l normal f a u l t s : down-throw t o e a s t . F i g u r e 4.11. T s i n g k a h t l e exposure: slump b a l l s w i t h i n the l a c u s t r i n e s i l t . 137 varve p a t t e r n i s masked by a s t r o n g l y developed j o i n t p a t t e r n t h a t r e s u l t e d from d r y i n g and c o n t r a c t i o n of the s i l t at T s i n g k a h t l e , F i g . 4.10. C o n t o r t i o n s w i t h i n the s i l t ( u n i t 2) appear to have r e s u l t e d from l o c a l slumping d u r i n g d e p o s i t i o n , F i g . .4.11. They are s i m i l a r t o f e a t u r e s observed w i t h i n the L i l l o o e t S i l t ( c f . 3.2.1 and 3.2.2). I n d i v i d u a l s t r u c t u r e s are s u r -rounded by u n d i s t u r b e d masses of s i l t , s u g g e s t i n g t h a t the slumping was i n i t i a t e d some d i s t a n c e u p - s l o p e from t h i s p a r t i c u l a r l o c a t i o n , the d e b r i s came to r e s t h e r e , then was promptly b u r i e d by more s i l t . S i n c e the c o n t o r t i o n s a l l occur a l o n g a p p r o x i m a t e l y the same h o r i z o n , i t appears t h a t there, was one d i s t i n c t e p isode of s l u m p i n g , p o s s i b l y i n i -t i a t e d i n the normal sequence of events when s i l t d e p o s i t i o n upon the i r r e g u l a r topography of the l a k e f l o o r c r e a t e d u n s t a b l e s l o p e s , or a l t e r n a t i v e l y , as the r e s u l t of an event such as a l a n d s l i d e or mudflow c a u s i n g a d i s t u r b a n c e i n some a d j a c e n t p a r t of the l a k e . S i m i l a r f e a t u r e s have been d e s c r i b e d by Kuenen (1948) and termed "slump b a l l s " . These are beds of i n t e r l a m i n a t e d s i l t and f i n e sand which- detached themselves from the t i p s of narrow a n t i c l i n e s or o v e r f o l d s as slumping o c c u r r e d . The observed p s e u d o - r i p p l e - m a r k s r e s u l t from the compression-of the i n n e r l a y e r s of the slump b a l l as the c o n t o r t i o n took p l a c e . 138 I n c l u d e d C o a r s e r M a t e r i a l ( U n i t s 3, 7 , 8 and 9) S e v e r a l beds b e l i e v e d to be of l o c a l r a t h e r than r e g i o n a l s i g n i f i c a n c e occur w i t h i n the l a m i n a t e d sand and s i l t . The g r a v e l s ( u n i t s 3 and 9) are t y p i c a l mudflow d e p o s i t s and p r o b a b l y o r i g i n a t e d from g u l l i e s i n a h i l l s i d e b o r d e r i n g t h i s s e c t i o n of the l a k e . Large l o a d c a s t s a l o n g the c o n t a c t between mudflow ( u n i t 9) and u n d e r l y i n g laminae ( u n i t 10) i n d i c a t e the impact of t h i s massive bed at the time of i t s d e p o s i t i o n , F i g . 4.12. The coarse, sand ( u n i t 8) l o c a t e d h i g h i n the sequence, and g r a v e l ( u n i t 7) are d e l t a i c sediments p r o b a b l y d e p o s i t e d by l o c a l streams--such as Venables C r e e k - - d u r i n g p e r i o d s of e x c e p t i o n a l l y h i g h f l o w . U n i t 1 i s a t y p i c a l t e r r a c e c a p p i n g of c o b b l e - g r a v e l such as veneers a l l r i v e r - c u t t e r r a c e s w i t h i n . t h e v a l l e y . I t shows an e r o s i o n a l c o n t a c t w i t h the u n d e r l y i n g s i l t , but the amount of s i l t removed must have been s m a l l , s i n c e t h i s t e r r a c e i s at the same e l e v a t i o n as the upper s u r f a c e of the s i l t where no e r o s i o n , has taken p l a c e . As i n d i c a t e d on F i g . 4.4, the T s i n g k a h t l e g u l l e y has been, p a r t i a l l y f i l l e d by mudflow and washed s i l t and then deepened s i n c e i t was o r i g i n a l l y eroded. A l t h o u g h the e n t i r e sequence d e s c r i b e d above i s o n l y exposed i n the T s i n g k a h t l e g u l l e y , the v a r i o u s u n i t s are commonly seen throughout the v a l l e y , F i g . 4.13. Exposures i n the s i l t ( u n i t 2) are the most widespread, and occur i n F i g u r e 4.12. T s i n g k a h t l e exposure: l o a d s t r u c t u r e s a l o n g c o n t a c t between mudflow g r a v e l and coarse sand. F i g u r e 4.14. Thompson V a l l e y near Basque: sand w i t h c u r r e n t r i p p l e - m a r k s at base of l a c u s t r i n e s i l t ; t e r r a c e - c a p p i n g of c o b b l e -g r a v e l above. FIGURE 4.13 GENERALIZED SEQUENCE OF UNCONSOLIDATED DEPOSITS IN THE THOMPSON VALLEY AEOLIAN SILT, SAND MUDFLOW GRAVEL (ALLUVIAL FAN) " ' \ ' » " ' o ' O * *» ° 0° . ° O ° O O . O O O . O ° o o o o o o o o o o a o o o » D » o o T » o o o 0 0 0 0 o TERRACE GRAVEL 3 0 0 -SCALE - FEET 8 SILT - grading to del taic gravels locally £ 2 0 0 -ac UJ > O ,50-0. 0. INTERLAMINATED SILT AND SAND 100-«> o o *> O o O o O - - K ^ . o ° " " • ° / \ «, o o X \ ° f * \ B A S A L GRAVEL 5 0 -• ' ° ° / x \ ° -* A > A \ + T I L L 0 - BEDROCK 141 the s c a r p s l o p e s of most, t e r r a c e s and i n the banks of many streams at e l e v a t i o n s below a p p r o x i m a t e l y 1 , 300;'. I n t e r -l a m i n a t e d s i l t and f i n e sand are found only i n a l i m i t e d , number of exposures t h a t are u s u a l l y l o c a t e d beneath the lowest r i v e r t e r r a c e , a s h o r t v e r t i c a l d i s t a n c e above p r e s e n t r i v e r l e v e l . C u r r e n t r i p p l e marks are an almost c o n s t a n t f e a t u r e h e r e , but c u t - a n d - f i l l s t r u c t u r e s were nowhere as w e l l developed as at T s i n g k a h t l e , F i g . 4.14. T h i s v a r i a t i o n c o u l d be due to the d i f f e r e n t r e l a t i v e p o s i -t i o n s of exposures w i t h i n the g e n e r a l sequence. W i t h i n t h i s s e c t i o n of the Thompson V a l l e y the upper l i m i t of the s i l t i s marked by a w e l l d e f i n e d c o n s t r u c t i o n a l bench. T h i s i s much more c o n t i n u o u s a l o n g the e a s t e r n than a l o n g the w e s t e r n v a l l e y s i d e , a l t h o u g h t h e r e are wide benches on the west bank near Spences B r i d g e and T s i n g k a h t l e F i g . 4.3. There i s no d i r e c t e vidence to i n d i c a t e whether the bench i s an o r i g i n a l f e a t u r e r e s u l t i n g from s i l t d e p o s i -t i o n a l o n g the margin of an i c e l o b e , or i f i t i s o n l y the remnant of a. f o r m e r l y much more e x t e n s i v e v a l l e y - f i l l . However, at lower e l e v a t i o n s the s i l t i s c l e a r l y seen t o have been eroded and. t e r r a c e d by the Thompson R i v e r , and so i t i s assumed t h a t the o r i g i n , of the bench i s s i m i l a r l y i due t o e r o s i o n . The s u r f a c e of the bench i s u n d u l a t i n g due t o com-p a c t i o n of the s i l t d u r i n g d r y i n g , the i r r e g u l a r topography o r i g i n a l l y b u r i e d , a n d l o c a l slumping and s l i d i n g a l o n g the p r e s e n t o u t e r edge of the bench. However, a g e n e r a l s o u t h e r 142 decrease i n e l e v a t i o n i s n o t e d , from about 1,400' near A s h c r o f t to l . , 250' near Spences B r i d g e , F i g . .4.15. T h i s c o u l d p o s s i b l y r e s u l t from the l o w e r i n g of the s u r f a c e l e v e l of Lake Thompson as i t extended southwards, or o t h e r -wise from i s o s t a t i c t i l t i n g - - r e q u i r i n g a t i l t of 6.8' per m i l e f o r the observed s l o p e - - w h i c h i s s i m i l a r to measure-ments made by Mathews (1944) from a d j a c e n t areas ( c f . 4 . 3 . 1 ) " f o l l o w i n g ) . To the n o r t h of A s h c r o f t the 1,400' s i l t bench i s r e p l a c e d by ( o r grades i n t o ) an a g g r a d a t i o n a l g r a v e l t e r r a c e at a s i m i l a r e l e v a t i o n t h a t i s p a r t of the e x t e n s i v e d e l t a of the Bonaparte R i v e r i n Lake Thompson. 4.2.2 D e l t a i c G r a v e l s The Spences B r i d g e s i l t i s found o n l y where the l a c u s t r i n e d e p o s i t i o n was f r e e from " c o n t a m i n a t i o n " from l o c a l s o u r c e s . C l o s e t o the mouths of t r i b u t a r i e s much g r a v e l was d e p o s i t e d as d e l t a s i n g l a c i a l Lake Thompson. These c o n s i s t e i t h e r of mixed s i l t and g r a v e l or of g r a v e l a l o n e , depending upon the regime of the d e p o s i t i n g streams. An example of the l a t t e r i s d e s c r i b e d below: The Twaal Creek D e l t a - F a n At the p o i n t of e n t r y of Twaal Creek i n t o Lake Thompson a l a r g e and complex d e l t a - f a n was formed .by a c o m b i n a t i o n of l a c u s t r i n e and s u b a e r i a l d e p o s i t i o n , F i g . 4.3. Twaal Creek i s one of the l a r g e r t r i b u t a r i e s of the Thompson i n t h i s a r e a , and a l s o f u n c t i o n e d as a g l a c i a l s p i l l w a y THOMPSON VALLEY - LONGITUDINAL PROFILE showing elevation of terraces, benches and alluvial fans 1,500 b a> 1,000 5 0 0 A A A A A _ A A _ approximate upper limit of silt " A „ A A ®£> A - — „ " ^ O O O j t bench _ -t BLACK CANYON SPENCES BRIDGE 10 15 2 0 ASHCROFT miles Compiled from altimeter traverse and air photo interpretation Note each symbol indicates the location of a feature but not its extent A - alluvial fan — - bench or terrace CD <Z zo m 144 d u r i n g the e x i s t e n c e of. the g l a c i a l l a k e , which p r o b a b l y accounts f o r the u n u s u a l l y l a r g e volume of m a t e r i a l d e p o s i t e d at i t s mouth. The Twaal d e l t a - f a n i s shaped l i k e a segment of a cone, w i t h the s u r f a c e s l o p i n g i r r e g u l a r l y " as a s e r i e s of s t e p s from a p p r o x i m a t e l y 1,900' at the apex to 1,350' at the o u t e r edge--the former l a k e l e v e l . P a r t of t h i s o u t e r edge now "hangs" 500' above the v a l l e y f l o o r , w h i l s t the remainder abuts a g a i n s t a low m o r a i n i c h i l l , i n d i c a t i n g t h a t d e p o s i t i o n of t h i s f e a t u r e may have commenced w h i l e i c e s t i l l o c c u p i e d the c e n t r a l p o r t i o n of the v a l l e y . The average g r a d i e n t i s 3 1/4° over a r a d i u s of s l i g h t l y more than a m i l e . The whole f e a t u r e has now been deeply d i s s e c t e d by the modern Twaal Creek and a s m a l l e r g u l l e y t o the s o u t h . The sequence of d e p o s i t s observed beneath the o u t e r edge of the d e l t a - f a n i s g i v e n i n Table 4.2. For i n t e r p r e t i v e purposes the 14 u n i t s t h a t were i d e n t i f i e d may be d i v i d e d i n t o t h r e e groups: b a s a l g r a v e l ( u n i t 1 4 ) ; d e l t a i c g r a v e l ( u n i t 1 3 ) ; " v a r i a b l e " s e r i e s of beds, i n c l u d i n g g r a v e l , sand and s i l t ( u n i t s 1 to 12). The B a s a l G r a v e l ( U n i t 14) T h i s massive grey g r a v e l i s b e l i e v e d to c o r r e s p o n d to the g r a v e l at the base of the T s i n g k a h t l e e x posure, F i g . 4.15. I t s c h a r a c t e r i s t i c s i n d i c a t e r a p i d d e p o s i t i o n c l o s e t o the i c e f r o n t , p o s s i b l y s u f f i c i e n t l y c l o s e f o r i t t o be c l a s s i f i e d as w a t e r l a i d moraine because t h i n beds of more t y p i c a l t i l l are a l s o p r e s e n t here. T h i s may then be TABLE 4.. 2 THE TWAAL DELTAIC SEQUENCE Top of S e c t i o n T h i c k n e s s ( f e e t ) U n i t 1. Mudflow g r a v e l w i t h l e n s e s of f i n e f l u v i a l g r a v e l . U n i t 2 U n i t 3 U n i t 4 U n i t 5 Non-sorted and p o o r l y bedded f l u v i a l g r a v e l becoming c o a r s e r upwards. A l t e r n a t i n g beds of s i l t and f l u v i a l g r a v e l , each 1.5' t o 2' t h i c k . C l a y - s i l t w i t h l a m i n a t i o n s and c o n t o r -t i o n s ; c o a r s e b o u l d e r bed a t base. 12 7 10 F i n e g r a v e l i n c l u d i n g l e n s e s of coar s e sand, 4 i n t e r b e d d e d w i t h n o n - s o r t e d f l u v i a l g r a v e l w i t h rounded c o b b l e s . U n i t 6. I n t e r b e d d e d co a r s e and f i n e sand w i t h v a r i o u s f l o w and l o a d s t r u c t u r e s . U n i t 7. Bedded mudflow g r a v e l w i t h t h i n l e n s e s of reworked f l u v i a l m a t e r i a l . U n i t 8. I n t e r b e d d e d co a r s e sand ( w i t h some con-t o r t i o n s ) and mudflow g r a v e l ; c o a r s e ;' J.. sand shows some f o r e s e t bedding w i t h d i p s towards west-south-west. 14 U n i t 9. Non-sorted and non-bedded f l u v i a l g r a v e l c o n s i s t i n g of c o b b l e s and pebbles w i t h much f i n e m a t r i x ; i n c l u d e s a 2' l e n s e of •foreset beds . 12 U n i t 10. T h i n mudflow beds a s s o c i a t e d w i t h beds of c o n t o r t e d sand. U n i t 11 U n i t 12 U n i t 13 Massive mudflow g r a v e l C o n t o r t e d s i l t . 20 25 U n d i f f e r e n t i a t e d d e l t a i c g r a v e l s i n c l u d i n g 100 t h i n beds of sand and s i l t , c r o s s - b e d d e d , moderate s o r t i n g ; i n c l u d e s o c c a s i o n a l beds of c o a r s e , a n g u l a r r h y o l i t e . , from a d j a c e n t o u t c r o p . 200 Table 4.2 ( c o n t i n u e d ) „ j r C J - - T h i c k n e s s Top of S e c t i o n ( f e e t ) U n i t 14. Coarse, n o n - s o r t e d , rounded, grey 40+ f l u v i a l g r a v e l w i t h rough s t r a t i f i c a t i o n d i p p i n g eastwards at a p p r o x i m a t e l y 10°; i n c l u d e s n o n - s o r t e d g r a v e l w i t h much c l a y -s i l t m a t r i x ( t i l l ) near base. variable series deltaic gravel basal gravel 'variable series" deltaic gravel and sand basal gravel F i g u r e 4.16. Twaal d e l t a i c sequence . F i g u r e 4.16. Twaal d e l t a i c sequence. 148 a l o c a l form of a b l a t i o n moraine, which was the g e n e r a l mode of o r i g i n suggested e a r l i e r f o r the widespread b a s a l g r a v e l beds. The l o c a l o c c u r r e n c e of i c e - f r o n t g r a v e l s here would a g a i n i n d i c a t e t h a t at l e a s t p a r t of the o v e r l y i n g d e l t a i c beds may have been d e p o s i t e d i n an i c e - m a r g i n a l l a k e t h a t was the f o r e r u n n e r of the main p r o - g l a c i a l l a k e as was sug-ge s t e d above. D e l t a i c G r a v e l T h i s i s a r a t h e r t y p i c a l d e l t a i c sequence r e s u l t i n g from c o n d i t i o n s of r a p i d d e p o s i t i o n , F i g . 4.16. The beds of v a r y i n g d i r e c t i o n s and a n g l e s of d i p are the f o r e s e t s e r i e s ( w h i l s t the t o p s e t beds are r e p r e s e n t e d by the " v a r i a b l e " sequence above). I s o l a t e d l a r g e b o u l d e r s w i t h i n the d e l t a i c beds were presumably d e p o s i t e d by i c e - r a f t i n g s u g g e s t i n g t h a t t h e r e w a s e i t h e r s t i l l g l a c i e r - i c e i n the v i c i n i t y , or t h a t the c l i m a t e was s u f f i c i e n t l y severe f o r a t h i c k l a y e r of i c e to form over the l a k e i n w i n t e r . " V a r i a b l e " S e r i e s T h i s c o n s i s t s of a m i x t u r e of beds of l a c u s t r i n e , f l u v i a l and mudflow o r i g i n s . The l a c u s t r i n e beds i n c l u d e s i l t s , i n t e r l a m i n a t e d s i l t and sands and cross-bedded sands and g r a v e l s ( u n i t s 4, 6, 8, 12 and p a r t s of 3, 8, and 10). F l u v i a l g r a v e l s are c h a r a c t e r i s t i c a l l y f r e e of f i n e m a t r i x , w e l l rounded and d i s p l a y v a r i o u s degrees of s o r t i n g and bedding ( u n i t s 2, 5, 9 and p a r t s of 1, 3 and 7 ) . Mudflow g r a v e l s are massive and d i s t i n g u i s h e d by t h e i r l a c k of s o r t i n g and the presence of much f i n e - g r a i n e d "mud" m a t r i x ( u n i t s 149 1, 7,. 11 and p a r t s of 8 and 1 0 ) . The i n t e r b e d d i n g of l a c u s t r i n e and s u b - a e r i a l ( f l u v i a l and mudflow) m a t e r i a l suggests t h a t l a k e l e v e l was f l u c t u a t i n g over a v e r t i c a l d i s t a n c e of about 80' ( a p p r o x i -mately 1,270' - 1,350' a . s . l . ) w h i l s t t h i s d e p o s i t i o n took p l a c e . S i n c e the l a k e was dammed by a g l a c i e r , some v a r i a -t i o n i n the l e v e l of the dam may be expected as the r e s u l t of s e a s o n a l v a r i a t i o n s i n the amount of m e l t i n g and s u p p l y of i c e . Many of the s m a l l - s c a l e s t r u c t u r e s w i t h i n the " v a r i a b l e " s e r i e s p o s s i b l y r e s u l t from d i s t u r b a n c e s caused by the a l t e r n a t i n g c o n d i t i o n s , i n p a r t i c u l a r , the w i d e s p r e a d slumping of l a c u s t r i n e s i l t , F i g . 4.17, 4.18. 4.2.3 Other L a c u s t r i n e D e p o s i t i o n The above two examples of d e p o s i t i o n i n Lake Thompson, the Spences B r i d g e s i l t at T s i n g k a h t l e and the d e l t a i c g r a v e l a t Twaal Creek, may be thought of as r e p r e s e n t i n g the o p p o s i t e ends of a g r a d a t i o n a l sequence between (1) pro-g l a c i a l l a k e d e p o s i t i o n r e s u l t i n g from an i n c r e a s i n g l y remote i c e - f r o n t , and (2) p r o g l a c i a l and m a r g i n a l l a k e d e p o s i t i o n r e s u l t i n g s o l e l y from a l o c a l " l a t e r a l " s o u r c e . V a r i o u s i n t e r m e d i a t e s i t u a t i o n s between these two end members are to be found at the mouths of most of the t r i b u t a r y c r e e k s i n the a r e a . At P i m a i n u s , P u k a i s t and I n k i k u h C r e e k s , l a c u s t r i n e d e p o s i t i o n i n c l u d e d both s i l t and g r a v e l . At Pimainus Creek the sequence c o n s i s t s c h i e f l y of bedded s i l t y g r a v e l F i g u r e 4.17. Twaal d e l t a i c sequence: con-t o r t i o n due to slumping of s i l t i n upper " v a r i a b l e " p a r t of the d e l t a i c sequence. F i g u r e 4.18. Twaal d e l t a i c s t r u c t u r e w i t h i n coarse and upper p a r t of sequence. sequence: f i n e sand f l o w -i n 151 w i t h o c c a s i o n a l , i s o l a t e d beds of pure s i l t . The s i l t y g r a v e l i s w e l l i n d u r a t e d and r e l a t i v e l y r e s i s t a n t to ero-s i o n , F i g . 4.19. At the mouth of P u k a i s t Creek i n t e r b e d d e d g r a v e l , s i l t , and s i l t y g r a v e l are o v e r l a i n by a massive s i l t y g r a v e l . Above t h i s are i n t e r b e d d e d c o b b l e g r a v e l , s i l t and,, mudflow g r a v e l which p o s s i b l y r e s u l t e d from the same a l t e r n a t i o n of c o n d i t i o n s as d e s c r i b e d f o r the upper p a r t of the Twaal sequence. In most creek-mouth exposures the p r o p o r t i o n of g r a v e l i n the sequence n o t i c e a b l y i n c r e a s e s towards the creek,, w h i l s t the c l e a n e s t s i l t beds are l o c a t e d c l o s e s t to the c e n t e r of the v a l l e y and at the g r e a t e s t d i s t a n c e from the mouth of the t r i b u t a r y . 4. 3 Events F o l l o w i n g the Drainage of Lake Thompson 4.3.1 The E s t a b l i s h m e n t of S o u t h e r l y Drainage Lake Thompson f i n a l l y d r a i n e d when the i c e dam' i n the v i c i n i t y of Spences B r i d g e gave way. The Spences B r i d g e s i l t d i s a p p e a r s a b r u p t l y here and i s r e p l a c e d by a t h i c k s e r i e s of f l u v i a l g r a v e l s ( A n d e r t o n , i n p r e p a r a t i o n ) . Thus the g e n e r a l p o s i t i o n of the i c e dam i s i n d i c a t e d — a b o u t 3 m i l e s t o the south of Spences B r i d g e - - b u t no d i r e c t e vidence of the dam i t s e l f , such as moraine, has been found t o date. Presumably the b r e a c h i n g of the dam was e i t h e r the case of an i s o l a t e d i c e - b l o c k f i n a l l y c o l l a p s i n g or f l o a t i n g under p r e s s u r e from the l a k e , or the i c e - f r o n t g r a d u a l l y melted back towards the F r a s e r V a l l e y . However, s i n c e t h e r e i s F i g u r e 4.19. Exposure at mouth of Pimainus Creek: showing i n t e r r e l a t i o n s h i p of d e l t a i c g r a v e l and l a c u s t r i n e s i l t . F i g u r e 4.19. Exposure at mouth of Pimainus of d e l t a i c g r a v e l and l a c u s t r i n e s i l t . Creek: showing i n t e r r e l a t i o n s h i p 153 no e vidence t h a t a major l a k e ever o c c u p i e d the Thompson V a l l e y between 3 m i l e s south of Spences B r i d g e and L y t t o n , i t may be supposed t h a t whichever of the above cases i s t r u e , the i c e no l o n g e r formed a w a t e r - t i g h t b a r r i e r once i t r e t r e a t e d from the v i c i n i t y of Spences B r i d g e . A l s o , a f t e r s o u t h e r l y d r a i n a g e commenced, i t i s l i k e l y t h a t the s i z e of the d r a i n a g e c h a n n e l was r a p i d l y e n l a r g e d by m e l t i n g of the i c e , and c o n s e q u e n t l y l a k e l e v e l f e l l r a p i d l y . In the L y t t o n - S p e n c e s B r i d g e s e c t i o n of the v a l l e y , the e s t a b l i s h m e n t of s o u t h e r l y d r a i n a g e was f o l l o w e d by a p e r i o d of a g g r a d a t i o n . T h i s i s i n d i c a t e d a t p r e s e n t by the s e v e r a l hundred f e e t of g r a v e l s and sand exposed a l o n g the steep r i v e r banks h e r e . The s u r f i c i a l and t o p o g r a p h i c f e a t u r e s t h a t e x i s t e d at the time of l a k e d r a i n a g e have been l a r g e l y obscured by l a t e r e v e n t s , but i t i s p r o b a b l y t h a t d r a i n a g e of the l a k e exposed an u n d u l a t i n g s u r f a c e , c h i e f l y of s i l t , but w i t h g r a v e l d e l t a s at creek mouths. ^ , G r a d i e n t s a l o n g the v a l l e y f l o o r would be s m a l l , and e i t h e r towards the n o r t h or south a c c o r d i n g to the l o c a l p a t terns'o'f a g g r a d a t i o n . I t i s i n t e r e s t i n g to s p e c u l a t e upon the mode of d r a i n a g e r e v e r s a l which must have taken p l a c e at t h i s time from a n o r t h e r l y d r a i n i n g l a k e , to a s o u t h e r l y d r a i n i n g l a k e f o l l o w e d by a s o u t h e r l y f l o w i n g stream. The d r a i n a g e d i r e c t i o n as the l a k e d r a i n e d would have been dependent upon the r e l a t i v e e l e v a t i o n s of the s u r f a c e s of l a c u s t r i n e 154 sediments. These were p r o b a b l y h i g h e s t at p o i n t s where the l a k e s u r f a c e had stood at the h i g h e s t l e v e l and where the g r e a t e s t amount of l a c u s t r i n e d e p o s i t i o n had taken p l a c e . High p o i n t s of the l a k e f l o o r would then i n c l u d e the area of the South Thompson S i l t t o the e a s t of Kamloops, w i t h a s u r f a c e p r e s e n t l y marked by a r l , 500' bench but a l r e a d y d i s s e c t e d d u r i n g the 1,400' stage of Lake Thompson by the e a s t e r l y f l o w i n g d r a i n a g e stream (Mathews, 1944; F u l t o n , 1965). Other h i g h p o i n t s were p r o b a b l y l o c a t e d at the mouths of the Deadman and Bonaparte R i v e r s where l a r g e d e l t a s were c o n s t r u c t e d i n t o the l a k e . One of t h e s e may have formed the watershed between d r a i n a g e to the F r a s e r and d r a i n a g e to the Okanagan at t h i s t i m e . S o u t h e r l y d r a i n a g e was p r o b a b l y i n i t i a t e d by the m i g r a t i o n upstream of a k n i c k p o i n t o r i g i n a t i n g at the s o u t h e r n o u t l e t of Lake Thompson when t h a t l a k e d r a i n e d . Both s o u t h e r l y d r a i n a g e and i n c r e a s e d d o w n c u t t i n g would have been f a v o u r e d by the increment of s o u t h e r l y r e g i o n a l s l o p e g a i n e d from i s o s t a t i c rebound. A component of 7' per m i l e towards the southwest f o r l a t e - G l a c i a l and p o s t - G l a c i a l t i l t i n g has been r e c o r d e d by Mathews (1944, p. 45) f o r the s h o r e l i n e of g l a c i a l Lake M e r r i t t i n the N i c o l a B a s i n to the e a s t of the Thompson V a l l e y . T h i s r e p r e s e n t s a g r a d i e n t of about 6' per m i l e a l o n g the g e n e r a l t r e n d of the Thompson V a l l e y which i s o n l y s l i g h t l y l e s s / than the p r e s e n t g r a d i e n t of 8' per m i l e a l o n g the Thompson 155 R i v e r . .. I f t h i s component were removed, the r i v e r between A s h c r o f t and Spences B r i d g e would have a g r a d i e n t of o n l y 2' per m i l e . Even though t h i s i s p r o b a b l y a maximum v a l u e , i t does appear t h a t the t i l t i n g of the l a n d may have had c o n s i d e r a b l e e f f e c t upon the e s t a b l i s h m e n t of r e g i o n a l d r a i n a g e p a t t e r n s d u r i n g e a r l y p o s t - G l a c i a l time when i s o -s t a t i c rebound was most a c t i v e . 4.3.2 F o r m a t i o n of R i v e r T e r r a c e s The e s t a b l i s h m e n t of a t h r o u g h - f l o w i n g stream was f o l l o w e d by a phase of r i v e r d o w n c u t t i n g and d i s s e c t i o n of the b o d i e s of l a c u s t r i n e s i l t and g r a v e l . T e r r a c e s were cut between the e l e v a t i o n s of the upper l i m i t of l a c u s t r i n e d e p o s i t i o n (marked by the u n d u l a t i n g s i l t bench) and a s e r i e s of broad t e r r a c e s a p p r o x i m a t e l y 150' above p r e s e n t r i v e r l e v e l ( t h a t i s , between 1,400' and 1,100' near A s h c r o f t , d e scending to between 1,250' and 870' near Spences B r i d g e ) , F i g . 4.15. The lower broad t e r r a c e s w i l l be r e f e r r e d to j o i n t l y as the "1,000 bench" and mark a p e r i o d of r e l a t i v e s t a b i l i t y at the end of the d o w n c u t t i n g phase d u r i n g which the v a l l e y f l o o r was widened. P o s s i b l y b a s e - l e v e l at t h i s time was c o n t r o l l e d by a g g r a d a t i o n t a k i n g p l a c e e i t h e r f u r t h e r downstream i n the Thompson V a l l e y or i n the F r a s e r V a l l e y . The 1,000' bench i s b e s t developed a l o n g the v a l l e y between A s h c r o f t and T s i n g k a h t l e , F i g . 4.3. The t e r r a c e s are cut i n t o the l a c u s t r i n e s i l t , which i s g e n e r a l l y F i g u r e 4.20. View northwards near Basque: e r o s i o n a l t e r r a c e s cut i n t o the l a c u s t r i n e s i l t and obscured by development of a l l u v i a l f a n . F i g u r e 4.22. Exposure at mouth of Oregon Jack Creek showing l a c u s t r i n e s i l t o ver-l y i n g t e r r a c e g r a v e l . THOMPSON VALLEY - TOPOGRAPHY AND UNCONSOLIDATED DEPOSITS 2<000 -j W <0 ci liJ 1500-i.oooH (A ) 15 miles north of Spences Bridge 1,000 Bench ^ terroce gravel 2 .000- , - ISOOH 10004 (B) 135 miles north of Spences Bridge 'A mudflow gravel terrace gravel CD d JO m ONE MILE VERTICAL EXAGGERATION 2 1 x | \ ) 158 exposed i n the s c a r p s l o p e s between t e r r a c e s , and capped by a l a y e r o f coa r s e f l u v i a l g r a v e l ( c o b b l e - g r a v e l ) t h a t i s up to 40' i n t h i c k n e s s ; t h i s r e p r e s e n t s the normal r i v e r -bed f i l l r a t h e r than a g g r a d a t i o n . I t r e s t s unconformably on the s i l t w i t h o l d c h a n n e l - c u t s commonly v i s i b l e a l o n g the u n d u l a t i n g s i l t - g r a v e l c o n t a c t , F i g s . 4.20, 4.13. The t e r r a c e cappings c o n s i s t o f moderately w e l l s o r t e d and bedded w e l l - r o u n d e d c o b b l e s and p e b b l e s . They are commonly i m b r i c a t e d i n d i c a t i n g f l o w to the sou t h . The g r a v e l out-crops a l o n g t e r r a c e edges are conspicuous i n the f i e l d and the g r a v e l f r e q u e n t l y veneers and masks the s i l t o u t c r o p beneath. Above the t e r r a c e g r a v e l a t h i n cover of a e o l i a n s i l t and sand b l a n k e t s the t e r r a c e s u r f a c e and reaches a maximum of 3' i n t h i c k n e s s except where i t t a k e s the form of dunes up to 10' i n h e i g h t . A g g r a d a t i o n D u r i n g T e r r a c i n g There i s s t r a t i g r a p h i c and t o p o g r a p h i c e v i d e n c e t o i n d i c a t e t h a t a g g r a d a t i o n of the v a l l e y f l o o r from l a t e r a l s o u r c e s was a c t i v e d u r i n g the f o r m a t i o n of the 1,000' bench. A l l u v i a l f a n s were b u i l t upon the broad t e r r a c e s c h i e f l y by mudflows ( c f . 10.1.3 f o l l o w i n g ) t h a t o r i g i n a t e d from both l a r g e and s m a l l t r i b u t a r y v a l l e y s , F i g s . 4.13, 4.19. T h i s a g g r a d a t i o n may have been s u f f i c i e n t l y r a p i d t o dam the main r i v e r arid c r e a t e temporary l a k e s . An example of such a l a k e was i d e n t i f i e d near the p r e s e n t c o n f l u e n c e of Oregon Jack Creek and the Thompson R i v e r . Below the 1,000' 159 bench here an exposure shows, the f o l l o w i n g sequence of beds Top of S e c t i o n T h i c k n e s s ( f e e t )^ 1. W e l l bedded w h i t e s i l t 1 0 - 1 5 2. Lenses of w e l l - r o u n d e d c o b b l e g r a v e l 0 - 2 0 Unc o n f o r m i t y 3. Massive w h i t e s i l t 20 •4. Bedded grey g r a v e l l y s i l t 70 + The g r a v e l l y s i l t ( u n i t 4) and the s i l t ( u n i t 3) r e p r e s e n t the normal l a c u s t r i n e (Lake Thompson) d e p o s i t i o n h e r e , and the unconformably o v e r l y i n g t e r r a c e - c a p p i n g of c o b b l e - g r a v e l i s a l s o a common arrangement, F i g . 4.2 2.. However, the presence of an upper s i l t ( u n i t 1) i s u n u s u a l , and i n t e r p r e t e d as i n d i c a t i n g the o c c u r r e n c e of a second l a k e phase i n t h i s s e c t i o n of the v a l l e y , p o s t - d a t i n g the f o r m a t i o n of the 1,000' bench. T h i s i s r e f e r r e d t o as "Oregon Jack Lake" a f t e r the creek where i t was l o c a t e d . P o s s i b l y i t was the r a p i d l y a g g r a d i n g a l l u v i a l f a n s upon t h 1,000' bench near Spatsum t h a t c r e a t e d the dam f o r t h i s l a k F i g . 4.3. That Oregon Jack Lake was contemporary w i t h the con-s t r u c t i o n of a l l u v i a l f a n s i s i n d i c a t e d by the presence of the Mazama v o l c a n i c ash i n both. I t was found w i t h i n the l a c u s t r i n e s i l t 4' below the p r e s e n t ground s u r f a c e and w i t h i n the mudflow g r a v e l s o f the a l l u v i a l fans at depths r a n g i n g from 4' to 22' ( c f . 10.3 ;) . T h i s w.ould i n d i c a t e t h a t Oregon Jack Lake was i n e x i s t e n c e d u r i n g the Mazama 160 ash f a l l , 6 , 600 y e a r s B..P. Subsequent Downcutting A f i n a l phase of d o w n c u t t i n g by the Thompson R i v e r p o s t - d a t e s Oregon Jack Lake, most a l l u v i a l f a n a g g r a d a t i o n and 6,600 y e a r s B.P. D u r i n g t h i s time the r i v e r was lowered by 150' to i t s p r e s e n t e l e v a t i o n , narrow r i v e r t e r r a c e s were cut a t e l e v a t i o n s below the 1 , 000' bench ( F i g . 4-. 15) and c o r r e s p o n d i n g d o w n c u t t i n g by many t r i b u t a r y streams r e s u l t e d i n the d i s s e c t i o n of o l d e r t e r r a c e s and many a l l u v i a l f a n s . 4.4 Chronology f o r the Thompson V a l l e y between A s h c r o f t  and Spences B r i d g e On the b a s i s of i n f o r m a t i o n g i v e n above, the f o l l o w i n g sequences of events can be d i s t i n g u i s h e d i n the e v o l u t i o n of the p r e s e n t l a n d s c a p e : Stage 1 D u r i n g the P l e i s t o c e n e g l a c i a t i o n a t h i n l a y e r of t i l l was d e p o s i t e d over most of the upland a d j a c e n t to the v a l l e y , and an unknown t h i c k n e s s of t i l l on the v a l l e y f l o o r . The l a t t e r p r o b a b l y c o n s i s t s i n p a r t of w a t e r - l a i d a b l a t i o n moraine. I n s u f f i c i e n t e v i d e n c e i s a v a i l a b l e at p r e s e n t to d e c i p h e r any d e t a i l s of g l a c i a l h i s t o r y . Stage 2 Ice p o s s i b l y remained i n the c e n t e r of the v a l l e y w h i l s t the v a l l e y s i d e s became i c e f r e e . Drainage from the 161 uplands and m e l t w a t e r f l o w e d v i a a connected s e r i e s of m a r g i n a l l a k e s and s p i l l w a y s a l o n g the edge of the v a l l e y i c e ( c f . 2.2.2). D e p o s i t i o n of d e l t a s and d e l t a - f a n s commenced m a r g i n a l t o the i c e l o b e . Stage 3 The v a l l e y i c e melted from n o r t h t o south and was r e p l a c e d by p r o g l a c i a l Lake Thompson which extended t h r o u g h -out the e n t i r e v a l l e y . The Spences B r i d g e s i l t , d e l t a i c g r a v e l s and m a t e r i a l i n t e r m e d i a t e between these two was d e p o s i t e d t hroughout the l e n g t h of the l a k e . Stage 4 F u r t h e r m e l t i n g of i c e i n the lower Thompson V a l l e y and F r a s e r V a l l e y brought about the d r a i n a g e of Lake Thompson and s o u t h e r l y f l o w of the Thompson R i v e r . Down-c u t t i n g by the Thompson R i v e r d i s s e c t e d and t e r r a c e d the e a r l i e r l a c u s t r i n e d e p o s i t s . Stage 5 A p e r i o d of r e l a t i v e s t a b i l i t y of base l e v e l f o l l o w e d d u r i n g which the p r e s e n t 1,000' bench formed the v a l l e y f l o o r and l a t e r a l a g g r a d a t i o n , c h i e f l y i n the form of a l l u v i a l f a n s , r e s u l t e d i n the ponding of temporary l a k e s . T h i s stage l a s t e d u n t i l a f t e r the d e p o s i t i o n of the.-Mazama v o l -c a n i c a s h , 6,600 y e a r s B.P. Stage 6 Renewed d o w n c u t t i n g by the Thompson R i v e r t o i t s p r e s e n t p o s i t i o n was the f i n a l event i n the f o r m a t i o n of the modern l a n d f orm.s . CHAPTER V THE BONAPARTE VALLEY--DEVELOPMENT OF CONTEMPORARY LANDFORMS 5.1 I n t r o d u c t i o n Between Cache Creek and C a r q u i l l e the Bonaparte R i v e r o c c u p i e s a b r o a d , f l a t - f l o o r e d v a l l e y i n c i s e d below the u n d u l a t i n g s u r f a c e of the Thompson s e c t i o n of the I n t e r i o r P l a t e a u . L o c a l r e l i e f i s l e s s than i n most of the o t h e r major v a l l e y s of the I n t e r i o r P l a t e a u , b e i n g between 2,000' and 3,000'. T h i s p a r t i a l l y r e f l e c t s the absence of r e c e n t d o w n c u t t i n g by the Bonaparte R i v e r which has l e f t the p o s t -G l a c i a l a g g r a d a t i o n a l f l o o r of the v a l l e y u n d i s s e c t e d at the p r e s e n t t i m e . In t h i s r e s p e c t t h i s a rea i s d i s t i n g u i s h e d from the F r a s e r and Thompson V a l l e y s where severe down-c u t t i n g has r e c e n t l y t a k e n p l a c e . The v a l l e y i s b o r d e red on the west by the T r a c h y t e H i l l s and on the east by the Arrowstone H i l l s w i t h both areas r i s i n g t o g e n t l y rounded summits at about 4,500'. The uplands are composed c h i e f l y of r o c k s of the Cache Creek (group, g r e e n s t o n e s , c h e r t , a r g i l l i t e , c h l o r i t e , q u a r t z - m i c a s c h i s t and minor l i m e s t o n e and q u a r t z i t e , which are capped by T e r t i a r y l a v a s o f the Kamloops group on the Arrowstone H i l l s ( D u f f e l l and McTaggart, 1952). However, bedrock o u t c r o p s are i n f r e q u e n t and c o n f i n e d t o the n o r t h -e a s t e r n p a r t of the r e g i o n ( F i g . .5.1) and to the g u l l i e s BONAPARTE VALLEY - LANDFORMS 164 of the more deeply i n c i s e d streams. The p r e s e n t landforms are p r i m a r i l y r e l a t e d t o the p a t t e r n of Quaternary, d e p o s i t s . 5 . 2 G l a c i a l D e p o s i t i o n G l a c i a l t i l l veneers much of the upland and a l s o occurs a l o n g the v a l l e y s i d e s . A w e l l d e f i n e d but u n d u l a t i n g bench at a p p r o x i m a t e l y 2,000' appears to be composed of t i l l . I t i s a prominent f e a t u r e a l o n g the western v a l l e y s i d e where i t can be t r a c e d almost c o n t i n u o u s l y between C a r q i l l e and Cache Creek, F i g s . 5.1 and 5.2. Along the e a s t e r n v a l l e y s i d e the bench extends southwards from the v i c i n i t y of Fan 5 to Cache Creek, becoming broader and b e t t e r d e f i n e d s o u t h -wards, F i g . 5.3. On the wider s e c t i o n s of the e a s t e r n bench, s t r e a m l i n e d low r i d g e s appear to have r e s u l t e d from the moulding a c t i o n of moving i c e . However, s i n c e t h e r e are no exposures i n the m a t e r i a l of which the bench i s composed, i t i s p o s s i b l e t h a t i t i s a bedrock f e a t u r e t h a t has been o n l y m o d i f i e d by i c e movement and the d e p o s i t i o n of a s u r -f i c i a l l a y e r of t i l l . A s e r i e s of s m a l l road cutting'sttro the east of Fan 5 g i v e some i n d i c a t i o n of the n a t u r e of m a t e r i a l u n d e r l y i n g the scarp s l o p e a l o n g the o u t e r edge of the bench. T h i s c o n s i s t s c h i e f l y of f l u v i a l d e p o s i t s , e x t r e m e l y v a r i a b l e i n terms of g r a i n - s i z e and bedding c h a r a c t e r i s t i c s , and marked throughout by secondary s t r u c t u r e s such as c o n t o r t i o n s and f a u l t i n g t h a t p o s s i b l y were caused by. the movement of i c e , w h i l s t i n c o n t a c t w i t h the t e r r a c e scar.p. These are then HI floodplain kame terrace F i g u r e 5.2. Morphology of the e a s t e r n s i d e of the Bonaparte V a l l e y . F i g u r e 5.3. Morphology of the western s i d e of the Bonaparte V a l l e y . F i g u r e of the 5.3. Morphology of the western Bonaparte V a l l e y . s i d e 166 i n t e r p r e t e d as i c e - c o n t a c t d e p o s i t s l a i d down by mel t w a t e r streams of v a r i a b l e d i s c h a r g e i n the d e p r e s s i o n between i c e and t e r r a c e s c a r p d u r i n g a l a t e - G l a c i a l phase. That i c e was s t i l l p r e s e n t both d u r i n g and a f t e r the d e p o s i t i o n of these beds i s demonstrated by the presence of a l i m e s t o n e b o u l d e r about 30' i n diameter r e s t i n g upon these d e p o s i t s , F i g . 5.1, 5.3. The o n l y p o s s i b l e agent of d e p o s i t i o n f o r t h i s must have been g l a c i a l i c e . I f the 2,000' bench c o n s i s t s e n t i r e l y of t i l l , the o c c u r r e n c e of the d i s t o r t e d f l u v i a l m a t e r i a l a l o n g i t s o u t e r edge c o u l d p o s s i b l y i n d i c a t e t h a t a c t i v e i c e , o c c u p y i n g o n l y the c e n t r a l p o r t i o n of the v a l l e y , c o n t r i b u t e d t o the form-a t i o n of the bench, as a k i n d of kame t e r r a c e , between i c e and v a l l e y w a l l . 5 . 3 F l u v i o - G l a c . i a l D e p o s i t i o n At a s l i g h t l y l a t e r stage of d e g l a c i a t i o n , when i c e o c c u p i e d o n l y the l o w e r , c e n t r a l p a r t of the v a l l e y , kame t e r r a c e s were c o n s t r u c t e d between the i c e and the scarp s l o p e s of the t i l l benches, F i g . 5.1. A kame t e r r a c e at a p p r o x i m a t e l y 1,700' a l o n g the western v a l l e y s i d e i s the most c o n t i n u o u s of these f e a t u r e s but o t h e r s occur at s l i g h t l y h i g h e r e l e v a t i o n s , F i g . 5.4. C o n s e q u e n t l y , the v a l l e y s i d e s r i s e i n a s e r i e s of narrow .benches between the v a l l e y f l o o r and the t i l l bench. Where the kame t e r r a c e s are very narrow, as i s g e n e r a l l y the c a s e , they have been s e v e r e l y d i s s e c t e d by the h i l l s i d e g u l l i e s and almost o b l i t e r a t e d . These F i g u r e 5.4. G e n e r a l view northwards a l o n g the Bonaparte V a l l e y ; note kame t e r r a c e ( l e f t f o r e g r o u n d - m i d d l e d i s t a n c e ) and a l l u v i a l f a n s a l o n g both v a l l e y s i d e s . 168 f e a t u r e s are of too l i m i t e d an e x t e n t t o i n d i c a t e on F.,ig'. "';'5%1 , The s u r f a c e s of the kame t e r r a c e s are covered w i t h rounded pebbles and b o u l d e r s of many d i f f e r e n t l i t h o l o g i e s , but t h e i r i n t e r n a l s t r u c t u r e i s unknown s i n c e no exposures e x i s t . 5 . 4 P o s t - G l a c i a l D e p o s i t i o n P o s t - G l a c i a l a g g r a d a t i o n b u r i e d any l o w - l y i n g g l a c i a l d r i f t and b u i l t up the v a l l e y f l o o r to i t s p r e s e n t e l e v a t i o n . The depth of t h i s v a l l e y - f i l l and i t s c o m p o s i t i o n are not known. W e l l r e c o r d s f o r l o c a t i o n s a d j a c e n t t o the j u n c t i o n of the Trans Canada Highway and the Ca r i b o o Highway at Cache Creek g i v e a maximum t h i c k n e s s of 58' of g r a v e l and sand above bedrock. I t i s u n l i k e l y t h a t t h i s s u r p r i s i n g l y s h a l l o w depth of bedrock g i v e s a t r u e i n d i c a t i o n of the g e n e r a l s i t u a t i o n . I t may r e f l e c t the p r o x i m i t y of the w e l l s t o the v a l l e y s i d e or the presence of a bedrock s i l l i n the c o n s t r i c t e d s e c t i o n of the Bonaparte V a l l e y where Cache Creek i s l o c a t e d . Two m i l e s t o the n o r t h of Cache Creek w e l l s at the b r i d g e of the C a r i b o o Highway over the Bonaparte R i v e r r e c o r d no bedrock at 60'. P o s s i b l y d u r i n g the e a r l y stages of f l u v i a l aggrada-t i o n the Bonaparte R i v e r was f l o w i n g i n t o the 1,400' stage of g l a c i a l Lake Thompson which at t h a t time o c c u p i e d the Thompson V a l l e y at A s h c r o f t t c f . .2.2.1). S i n c e the f l o o r of the Bonaparte V a l l e y at p r e s e n t stands between 1,500' and 1,600' i n the area s t u d i e d , i t was not covered by t h i s 169 l a k e at l e a s t d u r i n g the l a t e r stages of a g g r a d a t i o n . However, i t i s p o s s i b l e t h a t at some e a r l i e r date an arm of Lake Thompson may have extended t h i s f a r n o r t h and been f i l l e d i n w i t h l a c u s t r i n e sediment. F l u v i a l a g g r a d a t i o n would have proceeded on top of the d e l t a so formed. S e v e r a l hundred f e e t of d e l t a i c s i l t s , sands and g r a v e l s are exposed by d i s s e c t i o n of the Bonaparte R i v e r below a k n i c k p o i n t l o c a t e d about 2 m i l e s south of Cache Creek. A l l u v i a l f a n s have been c o n s t r u c t e d a l o n g the margins of the v a l l e y at the mouths of t r i b u t a r y streams. They are a s s o c i a t e d w i t h the development of h i l l s i d e g u l l i e s c u t t i n g through t i l l , f l u v i o - g l a c i a l m a t e r i a l s and some bedrock. The l a t e r a l a g g r a d a t i o n from t r i b u t a r y streams and g u l l i e s was contemporary w i t h the g e n e r a l a g g r a d a t i o n of the v a l l e y and c o n t i n u e d f o r sometime a f t e r w a r d s , b u i l d i n g up the fans to t h e i r p r e s e n t forms, F i g s . 5.2, 5.3. The Mazama v o l c a n i c ash i s found w i t h i n most of these fans a t depths of up t o 8' below ground l e v e l , i n d i c a t i n g t h a t by 6,600 y e a r s B.P. a g g r a d a t i o n by the Bonaparte R i v e r had ceased but f a n con-s t r u c t i o n was s t i l l t a k i n g p l a c e . The t o e s of most fans have been trimmed by the mean-d e r i n g of the main r i v e r , so t h a t most of them end i n steep b l u f f s s t a n d i n g up to about 20' above r i v e r l e v e l . Exposures i n t hese b l u f f s show t h a t the fans, c o n s i s t c h i e f l y of mudflow g r a v e l s w i t h some i n t e r b e d d e d f l u v i a l m a t e r i a l . There i s no e v i d e n c e to suggest t h a t any mudflows have taken p l a c e r e c e n t l y . 170 As mentioned above, d i s s e c t i o n , such as has r e c e n t l y -deepened the Thompson and F r a s e r V a l l e y s , i s p r o g r e s s i n g -headwards on the Bonaparte R i v e r downstream from the area s t u d i e d . CHAPTER VI THE KAMLOOPS AREAS-DEVELOPMENT OF CONTEMPORARY LANDFORMS 6 .1 I n t r o d u c t i o n The Kamloops Area has r e c e i v e d the most a t t e n t i o n of a l l the areas under i n v e s t i g a t i o n i n t h i s t h e s i s , and con-s e q u e n t l y i t s Quaternary h i s t o r y i s w e l l documented. The p r i n c i p a l workers i n c l u d e Dawson (1878, 1895), Daly ( 1 9 1 5 ) , Mat.hews-:(1944) , F u l t o n (1 963 , 1965 ) and Armstrong and F u l t o n (1965). The f o l l o w i n g account i s m a i n l y a summary of p r e -v i o u s l y p u b l i s h e d work. The area i s u n d e r l a i n by P a l a e o z o i c sedimentary and v o l c a n i c r o c k s w i t h some s m a l l areas of g r a n i t i c i n t r u s i v e s of J u r a s s i c age ( C o c k f i e l d , 1948). However, the bedrock i s o n l y l o c a l l y i n c o r p o r a t e d i n t o the Quaternary v a l l e y l a n d -forms and i s not i m p o r t a n t ; the bedrock v a l l e y f l o o r i s b u r i e d by s e v e r a l hundred f e e t o f u n c o n s o l i d a t e d m a t e r i a l . The South Thompson R i v e r i s f l a n k e d by benches and t e r r a c e s t h a t are composed of u n c o n s o l i d a t e d Quaternary m a t e r i a l s , and which r i s e up to s l i g h t l y more than 1,500' (350' above r i v e r l e v e l ) . Behind the benches the bedrock v a l l e y s i d e s r e a c h to about 3,700' and c u l m i n a t e i n the r o l l i n g s u r f a c e of the I n t e r i o r P l a t e a u , g i v i n g a l o c a l r e l i e f t o t h i s a r ea of 2,600'. 172 6.2 P l e i s t o c e n e H i s t o r y The o l d e s t known P l e i s t o c e n e m a t e r i a l i n the area i s a g l a c i a l t i l l t h a t i s exposed a d j a c e n t to P e t e r s o n Creek about one m i l e to the south of Kamloops (Armstrong and F u l t o n , 1965), F i g . 6.1. The a b s o l u t e age of t h i s t i l l i s unknown, but i t i s o v e r l a i n by 90' of i n t e r g l a c i a l m a t e r i a l ( u n o x i d i z e d c l a y and s i l t g r a d i n g upwards i n t o o x i d i z e d s i l t and sand) c o n t a i n i n g wood t h a t was dated as o l d e r than 32,700 y e a r s B.P. (GSC-275, Dyck, F y l e s and B l a k e , 1965). The upper s u r f a c e of the i n t e r g l a c i a l beds i s e r o s i o n a l and marks a l o n g p e r i o d of s u b - a e r i a l c o n d i t i o n s d u r i n g the l a t t e r p a r t of the i n t e r g l a c i a l . Above t h i s i s a p r e -g l a c i a l sequence of c o n t o r t e d s i l t and sand capped by another t i l l s h eet. T h i s i n t u r n i s o v e r l a i n by p o s t - g l a c i a l beds of g l a c i o - l a c u s t r i n e s i l t , sand and g r a v e l (Armstrong and F u l t o n , 1965). Thus the sequence of events here may be r e c o n s t r u c t e d : "(1) e a r l y g l a c i a t i o n f o l l o w e d by g l a c i o - l a c u s t r i n e sedimen-t a t i o n ; (2) f o r m a t i o n and o x i d a t i o n of an i n t e r g l a c i a l l a c u s t r i n e s u c c e s s i o n ; (3) e r o s i o n ; (4) d e p o s i t i o n of u n o x i d i z e d , p o s s i b l y p r o - g l a c i a l m a t e r i a l ; (5) g l a c i a t i o n ( e q u i v a l e n t to the F r a s e r or C l a s s i c a l W i s c o n s i n ) f o l l o w e d by g l a c i o - l a c u s t r i n e depo s i t ion", (Arms t r ong and F u l t o n , 1965 , p. 100 ) . The P e t e r s o n Creek exposure i s the o n l y p l a c e where the l o w e s t beds of the P l e i s t o c e n e s u c c e s s i o n 1 have d e f i n i t e l y been i d e n t i f i e d . However, some o x i d i z e d sands FIGURE 6.1 KAMLOOPS AREA - SURFICIAL GEOLOGY AND LANDFORMS LEGEND PLEISTOCENE and RECENT Alluvial Stream channel and flood-plain deposits: gravel, sand and mucky peat Alluvial fan deposits: poorly sorted gravel, sand, silt and clay Lacustrine and Glacio - Lacustrine Lake deposits: chiefly silt with small amounts of clay, sand and gravel Fluvial and Glacio - Fluvial Stream deposits : gravel, sandy - gravel and sand Kettled stream deposits: gravel, sandy-gravel, sand and silt Glacial and Glacio - Fluvial Morainal gravels1 poorly sorted gravel and sand', irregular topography including kames and eskers Glacial Till Drift, undifferentiated Glacial and Interglacial Sub- t i l l stratified deposits1 includes ice advance outwash and non-glacial sand, silt and gravel TERTIARY and EARLIER | A A | Bedrock: includes areas of outcrop or near surface rock 174 and g r a v e l s , s i m i l a r to those at P e t e r s o n Creek, were l o c a t e d by the w r i t e r i n the n o r t h e r n bank of the South Thompson R i v e r between 2 and. 3 m i l e s to the e a s t of Kamloops. The g r a v e l beds here are f o r e s e t and are s t r o n g l y o x i d i z e d , con-t a i n i n g nodules of l i m o n i t e . Up to 6' of g r a v e l i s exposed j u s t above r i v e r l e v e l , but the base of the g r a v e l i s not seen. I f these c o r r e s p o n d to the P e t e r s o n Creek i n t e r -g l a c i a l beds then t h e i r e l e v a t i o n g i v e s a minimum v a l u e f o r the e l e v a t i o n of the v a l l e y f l o o r at t h i s p o i n t d u r i n g a l l subsequent ti m e . Thus the l a t e - g l a c i a l South Thompson s i l t must r e s t upon a v e r y i r r e g u l a r s u r f a c e s i n c e elsewhere i t extends t o over 100' below the p r e s e n t r i v e r l e v e l ( c f . 6.3). 6.3 D e g l a c i a t i o n The f i r s t f l u v i o - g l a c i a l d e p o s i t i o n took p l a c e upon the v a l l e y s i d e to the south of Kamloops w h i l s t i c e s t i l l o c c u p i e d the c e n t r a l p a r t of the v a l l e y ; many kame t e r r a c e s ( p i t t e d outwash t e r r a c e s ) were c o n s t r u c t e d ( F u l t o n , 1963). With f u r t h e r m e l t i n g g l a c i a l Lake Thompson was ponded i n the South Thompson V a l l e y between e a s t e r l y and w e s t e r l y r e t r e a t i n g i c e l o b e s ( c f . 2.2.1). The l a c u s t r i n e d e p o s i t s from Lake Thompson i n the Kamloops area today form much more prominent landforms than the a c t u a l s h o r e l i n e f e a t u r e s o f the l a k e . During, the e a r l y phases of l a c u s t r i n e d e p o s i t i o n i c e - c o n t a c t g r a v e l s were l a i d down. Some of t h e s e are exposed i n a g r a v e l p i t i n B a r n h a r t v a l e and are c o n t i n u o u s and t h e r e f o r e contemporary 175 w i t h hummocky g r a v e l kames. to the east ( F u l t o n , . 1965 ). S i m i l a r g r a v e l s are a l s o exposed to the n o r t h of. the r i v e r i n the v a l l e y of Scheidam Creek, F i g s . .6.1, ;6..;2. The major- form of d e p o s i t i o n i n Lake Thompson was the South Thompson s i l t ( F u l t o n , 1965). During s i l t d e p o s i -t i o n the l e v e l of the l a k e was between 1,650' (the h i g h e s t r e m a i n i n g o c c u r r e n c e of the s i l t ) and 1,750' ( t h e lo w e s t kame t e r r a c e ) s (Armstrong and F u l t o n , 1955). The s i l t u nconformably o v e r l i e s the i c e - c o n t a c t g r a v e l mentioned above; i t i n c l u d e s some slumped g r a v e l but i s o t h e r w i s e f r e e of c o a r s e r m a t e r i a l . The f o l l o w i n g d e s c r i p t i o n of the South Thompson s i l t i s p a r a p h r a s e d from F u l t o n (1965): 'The South Thompson S i l t i s a s t r a t i f i e d d e p o s i t t h a t o c c u p i e s the f l o o r o f the South Thompson V a l l e y and may be t r a c e d c o n t i n u o u s l y from Kamloops to a p o i n t 36 m i l e s t o the e a s t . The s i l t i s medium to coa r s e g r a i n e d , w h i t e i n dry exposures and l i g h t b l u e - g r e y i n u n o x i d i z e d c o n d i t i o n s . I t c o n t a i n s t h i n c l a y - r i c h bands which a c c e n t u a t e the s t r a t i f i c a t i o n and d e f i n e a p a t t e r n of v a r v e s , each v a r v e c o n s i s t i n g o f a t h i c k s i l t band and a t h i n n e r c l a y - r i c h band. The v a r v e s show a u n i f o r m i n c r e a s e i n t h i c k n e s s downwards, from 1" at the top of s e c t i o n s t o 250" at the base. ^ A l t h o u g h the e n t i r e - sequence i s not exposed at any one p l a c e , t h e r e i s an e s t i m a t e d t o t a l of 200 v a r v e s w i t h i n an e s t i m a t e d minimum t h i c k n e s s of 500' f o r the whole d e p o s i t . ' F u l t o n a s c r i b e s the s t r a t i f i c a t i o n w i t h i n the s i l t t o F i g u r e 6.2. Scheidam l a c u s t r i n e sediments--unconformably o v e r l a i n S i l t . Creek v a l l e y : g l a c i o -cross-bedded g r a v e l s by the South Thompson F i g u r e 6.3. Morphology of the South Thompson V a l l e y ; the wh i t e s i l t bench and s c a r p . 177 the c o n t r o l of the annual c l i m a t i c c y c l e upon d e p o s i t i o n , the t h i c k e r s i l t bands being- the summer d e p o s i t and the t h i n n e r c l a y bands,.the w i n t e r . V a r v i n g i n g l a c i o - l a c u s t r i n e sediments i s by no means u n u s u a l ; however, the t h i c k 15' var v e s near the base must be r e g a r d e d as e x c e p t i o n a l and have r e q u i r e d e x t r a o r d i n a r y c o n d i t i o n s f o r d e p o s i t i o n : -"The v a r v e d sediments were d e p o s i t e d i n a t r o u g h shaped b a s i n through which a l a r g e area i s d r a i n e d . At c e r t a i n s t a g e s d u r i n g the d e g l a c i a t i o n l a r g e q u a n t i t i e s of sediment-l a d e n m e l t w a t e r were f u n n e l l e d i n t o t h i s narrow t r e n c h . I t appears t h a t under such optimum c o n d i t i o n s as much as 20' of m a t e r i a l c o u l d be d e p o s i t e d i n a s i n g l e y e a r , " ( F u l t o n , 1965, p. 566). The s i l t was d e r i v e d by meltwater streams r e w o r k i n g the upland t i l l s h e e t s . The c o a r s e r f r a c t i o n remained behind as g r a v e l beds i n sediment t r a p s on the u p l a n d s , w h i l e the s i l t was ' c a r r i e d t o the v a l l e y s i n s u s p e n s i o n and d e p o s i t e d i n the l a k e ( F u l t o n , 1965). The s i l t grades westwards i n t o sand and g r a v e l near Kamloops, a l t h o u g h some extends f u r t h e r t o the. west beneath the v a l l e y f l o o r t o at l e a s t the e a s t e r n end of Kamloops Lake. W e l l l o g s examined by the w r i t e r show t h a t up to 250' of " s i l t y c l a y " o c c u r s beneath the Kamloops t o w n s i t e , 145' of " b l u e s i l t " and some sand beneath the western p a r t of Nor t h Kamloops and up to 160' of sand and s i l t beneath Kamloops a i r p o r t . These w e l l s are a l l i n the c e n t r a l . p a r t of the v a l l e y . C l o s e r t o the v a l l e y w a l l s g r a v e l s occur 178 w i t h i n the f i n e r m a t e r i a l between Kamloops and Kamloops Lake. P o s s i b l y the s i l t , sand and g r a v e l beds beneath the v a l l e y f l o o r were d e p o s i t e d i n Lake Thompson d u r i n g a s l i g h t l y l a t e r and lower stage of the l a k e than the South Thompson s i l t . 6 . 4 P o s t - G l a c i a l H i s t o r y D i s s e c t i o n of the s i l t f l o o r of Lake Thompson by an e a s t e r l y f l o w i n g stream took p l a c e d u r i n g a l a t e r and lower stage of the l a k e ( F u l t o n , 1963; c f . 2.2) w h i l s t i t was d r a i n i n g v i a c o l s between 1,100' and 1,200'. on the Thompson-Okanagan watershed. F i n a l l y , as the l a s t i c e m elted from the Thompson V a l l e y , the d r a i n a g e r e v e r s e d and the p r e s e n t westward p a t t e r n was e s t a b l i s h e d . A s m a l l amount of down-c u t t i n g s i n c e r e v e r s a l has formed some low r i v e r t e r r a c e s which stand up to 25' above p r e s e n t r i v e r l e v e l . The South Thompson s i l t p r e s e n t l y stands as benches on both s i d e s of the South Thompson V a l l e y , F i g . 6.3. The bench s u r f a c e s l o p e s downwards towards the c e n t e r of the v a l l e y from between 1,650' and 1,550' a d j a c e n t to the v a l l e y -s i d e s to between 1,500' and 1,400' at the o u t e r edge. The t r a n s v e r s e s l o p e i s a t t r i b u t e d by F u l t o n (1965) to d i f f e r e n -t i a l compaction and e r o s i o n r a t h e r than to the mode of d e p o s i t i o n . The d i s s e c t i o n and f o r m a t i o n of benches i n the South Thompson s i l t c r e a t e d a steep s c a r p face, up to. 300' i n 179 h e i g h t o v e r l o o k i n g the c e n t r a l p o r t i o n of the v a l l e y . G u l l i e s formed on the sca r p f a c e and expanded headwards i n t o the s i l t , d i s s e c t i n g the bench s t i l l . f u r t h e r . These g u l l i e s formed (and continue, t o grow today) p a r t l y by "normal" s u r f a c e e r o s i o n of the s i l t and p a r t l y by the s u b t e r r a n e a n p r o c e s s e s of " p s e u d o - k a r s t " which i n c l u d e the undermining and c o l l a p s e of w a t e r - s a t u r a t e d s i l t and t u n n e l l i n g ( P a r k e r , 1963; Buckham and C o c k f i e l d , 1950). S m a l l a l l u v i a l f a n s have been formed where the g u l l i e s t h a t are cut i n the s i l t bench debouch onto the lower t e r -r a c e . The f a n s are p o s s i b l y contemporary w i t h the i n i t i a l s tage of g u l l e y f o r m a t i o n and a l a t e r phase of g u l l y a l l u v i a -t i o n . At p r e s e n t , a f u r t h e r phase of d i s s e c t i o n i s r e -e x c a v a t i n g the f i l l i n the g u l l i e s and d i s s e c t i n g some p a r t s of the f a n s . Two l a y e r s of v o l c a n i c ash are p r e s e n t i n the f i l l of some g u l l i e s : the S t . Helen's "Y" ash from 3,200 y e a r s B.P. ( C r a n d e l l , M u l l i n e a u x , M i l l e r and R u b i n , 1962) and the Mazama ash from 6,600 y e a r s B.P. (Powers and W i l c o x , 1964). T h i s would i n d i c a t e t h a t a l l u v i a t i o n was t a k i n g p l a c e i n the g u l l i e s from b e f o r e 6,600 to a f t e r 3,200 yea r s ago ( F u l t o n and Armstrong, 1965). For a more d e t a i l e d d i s c u s s i o n of the p o s t - g l a c i a l s e d i m e n t a t i o n see S e c t i o n 10.1.5. ^CHAPTER V I I THE SIMILKAMEEN VALLEY--DEVELOPMENT OF CONTEMPORARY LANDFORMS 7 .1 I n t r o d u c t i o n From Keremeos to the I n t e r n a t i o n a l Boundary at Chopaka the Similkameen R i v e r o c c u p i e s a f l a t - f l o o r e d , t r o u g h - l i k e v a l l e y t h a t v a r i e s i n w i d t h between one and two m i l e s . I t i s f l a n k e d on the west by the most e a s t e r l y e x t e n s i o n of the Cascade Mountain system, the Okanagan Range where rounded summits r i s e to between 8,000' and 9,000' ( H o l l a n d , 1964). To the east of the v a l l e y i s the most s o u t h e r l y s e c t i o n of the Thompson P l a t e a u ( p a r t of the I n t e r i o r P l a t e a u ) w i t h an average summit e l e v a t i o n of 6,000' ( H o l l a n d , 1964). There i s a sharp break of s l o p e between both of these g e n t l y r o l l i n g u pland a r e a s , which are remnants of a T e r t i a r y e r o s i o n s u r f a c e , and the s t e e p e r v a l l e y s i d e s which date from entrenchment i n i t i a t e d d u r i n g the P l i o c e n e ( c f . 1 . 1 ) , F i g . 7.1. The Similkameen r e c e i v e s i t s l a r g e s t t r i b u t a r i e s from the west, namely Snehumption, Susap and Robert C r e e k s ; the l a r g e s t of western t r i b u t a r i e s are the o n l y ones to m a i n t a i n t h e i r f l o w d u r i n g the summer drought. The b a s i n s of the e a s t e r n t r i b u t a r i e s are c o n s i d e r a b l y s m a l l e r and have much s t e e p e r g r a d i e n t s , than those of the west bank, F i g . 7.1. FIGURE 7 . 2 SIMILKAMEEN VALLEY - BEDROCK GEOLOGY LEGEND MODERN 15 Recent alluvium, glacial drift TERTIARY Eocene 14 MARRON FORMATION: basalt lava, breccia, tuf f and conglomer-13 ate SPRINGBROOK FORMATION: conglomerate, sandstone, shale JURASSIC and/or Younger 12 Oliver and Cathedral granites II Granodiorite 10 Kruger, Oliver and Olalla syenites 9 Diorite JURASSIC ( ? ) , 8 . Richter Mountain hornblendite and Olalla pyroxenite 7 ( Osoyoos and Fairview granodiorite and associated rock types TRIASSIC or > Older 5 OLD TOM FORMATION: greenstone, basalt flows and sills, diorite 4 SHOEMAKER FORMATION: chert, tuff and greenstone 3 BARSLOW FORMATION: argillite PERMIAN 2 BLIND CREEK FORMATION: limestone CARBONIFEROUS : 1 KOBAU GROUP: quartzite, schist and greenstone 6 Altered rocks of diontic composition, origin uncertain 183 T h i s s e c t i o n of the Similkammen V a l l e y i s cut i n t o Mesozoic and e a r l i e r s t r a t i f i e d r o c k s which have been i n t r u d e d by J u r a s s i c and younger igneous s e r i e s , F i g . 7.2. The s t r a t i f i e d r o c k s predominate i n the n o r t h e r n p a r t of the a r e a : the e a s t e r n v a l l e y s i d e between B l i n d Creek and a p o i n t e i g h t m i l e s to the south i s u n d e r l a i n by the Kobau group, c o n s i s t i n g of C a r b o n i f e r o u s ( ? ) q u a r t z i t e , s c h i s t and gr e e n s t o n e ; t h e s e d i p westwards beneath the T r i a s s i c v o l c a n i c s of the Old Tom and Shoemaker Formations which out-crop on the western v a l l e y s i d e ( B o s t o c k , 1940), F i g . 7.2. South of Susap Creek both the e a s t e r n and western v a l l e y s i d e s are composed of J u r a s s i c i n t r u s i v e r o c k s which form the Similkameen b a t h o l i t h ( D a l y , 1912). These are c h i e f l y d i o r i t e w i t h some s y e n i t e ; d e b r i s d e r i v e d from here i s one of the major c o n s t i t u e n t s of a l l u v i a l fans on the f l o o r of the main v a l l e y . There are numerous s m a l l f a u l t s i n the a r e a ; i n f a c t Bostock (1930) suggests t h a t the t r e n d of the main v a l l e y I may be f a u l t - d e t e r m i n e d , and c e r t a i n l y the R i c h t e r V a l l e y appears to be f a u l t c o n t r o l l e d at l e a s t i n the s e c t i o n t o the s o u t h e a s t of R i c h t e r Mountain, F i g . 7.2. 7.2 G l a c i a l D e p o s i t s 7.2.1 L a t e r a l T i l l Sheet Remnants of a t i l l sheet are to be found b l a n k e t i n g the v a l l e y s i d e s i n many l o c a t i o n s , p a r t i c u l a r l y on the 184 e a s t e r n h i l l - s l o p e s , F i g . .7.1. The t i l l c o n s i s t s of pebble-to c o b b l e - s i z e d m a t e r i a l w i t h o c c a s i o n a l b o u l d e r s s e t i n a h i g h p r o p o r t i o n of w h i t e , s i l t y m a t r i x t h a t i s ve r y s i m i l a r i n appearance t o the w h i t e s i l t , t h a t comprises so much of the u n c o n s o l i d a t e d m a t e r i a l i n the v a l l e y s throughout the I n t e r i o r of B r i t i s h Columbia. Exposures i n the t i l l show s e v e r a l c h a r a c t e r i s t i c f e a t u r e s : t h e r e i s an apparent decrease i n the average s i z e of the l a r g e r p a r t i c l e s upwards; the p r o p o r t i o n of e r r a t i c m a t e r i a l i n c r e a s e s upwards; bedding i s g e n e r a l l y absent i n a l l but the b a s a l l a y e r s , F i g s . 7.3, 7.4. One sample of the t i l l was ta k e n from an ou t c r o p t o the s o u t h e a s t of Cawston, about 1,000' above the v a l l e y f l o o r . A n a l y s i s showed i t to c o n s i s t of 80% g r a v e l and 20% f i n e s by weight (see Appendix #1, S36). The f i n e s were composed of 39% sand, 33% s i l t and 28% c l a y , which suggests t h a t the t i l l may be the source of much of the f i n e - g r a i n e d m a t e r i a l t h a t was l a t e r i n c o r p o r a t e d as mudflow i n a l l u v i a l f a n s ( c f . 10.1.6 f o l l o w i n g ) . In t h i s a r e a the s u r f a c e form of the remnant t i l l s heet i s c h i e f l y the r e s u l t of r e c e n t e r o s i o n . In p l a c e s the t i l l s tands as p i l l a r s up to 30' h i g h , which are a l i g n e d i n n e a r l y p a r a l l e l rows r u n n i n g up and down the h i l l s i d e s w i t h the rows s e p a r a t e d by t a l u s streams from bedrock out-c r o p s above. Presumably these f e a t u r e s r e p r e s e n t the f i n a l stage of e r o s i o n which began as g u l l y i n g on the s u r f a c e of the t i l l s h e e t , F i g . 7.5. F i g u r e 7.3. L a t e r a l t i l l s h eet: b a s a l l a y e r -a n g u l a r b l o c k s of l o c a l bedrock w i t h s i l t m a t r i x . F i g u r e 7.4. L a t e r a l t i l l s h eet: upper s e c t i o n of t i l l "hoodoo"; note the predominance of rounded, e r r a t i c pebbles and c o b b l e s . F i g u r e 7.5. Morphology of l a t e r a l t i l l sheet r e s u l t i n g from e r o s i o n : "hoodoos" and p a r a l l e l g u l l i e s . F i g u r e 7.6. t i l l s h e e t : O r i g i n a l morphology of l a t e r a l l a t e r a l r i d g e and c h a n n e l . g u l l i e s F i g u r e 7.6. t i l l s h eet: O r i g i n a l morphology of l a t e r a l l a t e r a l r i d g e and c h a n n e l . 187 The p i l l a r s c h a r a c t e r i s t i c a l l y are l o c a t e d 1,000' or more above the v a l l e y f l o o r , but at lower e l e v a t i o n s e r o s i o n of the l a t e r a l t i l l sheet has been l e s s s e v e r e . Here, out-crops may be i d e n t i f i e d by p a t t e r n s of p a r a l l e l g u l l i e s r u n n i n g down the h i l l s i d e s owing t h e i r r e g u l a r i t y to the homogeneity of the t i l l . Rarely', the- upper l i m i t of t h i s t i l l i s marked by a narrow r i d g e , r u n n i n g p a r a l l e l t o the main v a l l e y but s t a n d i n g s e p a r a t e from the a d j a c e n t h i l l s i d e , w i t h the t y p i c a l form of a l a t e r a l moraine. S e v e r a l such r i d g e s were observed on the v a l l e y s i d e s west of Keremeos